Re: [Qemu-devel] [PATCH v6 1/7] linker-loader: Add new 'write pointer' command

[Ben Warren]

> On Feb 15, 2017, at 10:24 AM, Igor Mammedov <address@hidden> wrote:
> 
> On Wed, 15 Feb 2017 20:04:40 +0200
> "Michael S. Tsirkin" <address@hidden <mailto:address@hidden>> wrote:
> 
>> On Wed, Feb 15, 2017 at 06:43:09PM +0100, Igor Mammedov wrote:
>>> On Wed, 15 Feb 2017 18:39:06 +0200
>>> "Michael S. Tsirkin" <address@hidden> wrote:
>>> 
>>>> On Wed, Feb 15, 2017 at 04:56:02PM +0100, Igor Mammedov wrote:  
>>>>> On Wed, 15 Feb 2017 17:30:00 +0200
>>>>> "Michael S. Tsirkin" <address@hidden> wrote:
>>>>> 
>>>>>> On Wed, Feb 15, 2017 at 04:22:25PM +0100, Igor Mammedov wrote:    
>>>>>>> On Wed, 15 Feb 2017 15:13:20 +0100
>>>>>>> Laszlo Ersek <address@hidden> wrote:
>>>>>>> 
>>>>>>>> Commenting under Igor's reply for simplicity
>>>>>>>> 
>>>>>>>> On 02/15/17 11:57, Igor Mammedov wrote:      
>>>>>>>>> On Tue, 14 Feb 2017 22:15:43 -0800
>>>>>>>>> address@hidden wrote:
>>>>>>>>> 
>>>>>>>>>> From: Ben Warren <address@hidden>
>>>>>>>>>> 
>>>>>>>>>> This is similar to the existing 'add pointer' functionality, but 
>>>>>>>>>> instead
>>>>>>>>>> of instructing the guest (BIOS or UEFI) to patch memory, it instructs
>>>>>>>>>> the guest to write the pointer back to QEMU via a writeable fw_cfg 
>>>>>>>>>> file.
>>>>>>>>>> 
>>>>>>>>>> Signed-off-by: Ben Warren <address@hidden>
>>>>>>>>>> ---
>>>>>>>>>> hw/acpi/bios-linker-loader.c         | 58 
>>>>>>>>>> ++++++++++++++++++++++++++++++++++--
>>>>>>>>>> include/hw/acpi/bios-linker-loader.h |  6 ++++
>>>>>>>>>> 2 files changed, 61 insertions(+), 3 deletions(-)
>>>>>>>>>> 
>>>>>>>>>> diff --git a/hw/acpi/bios-linker-loader.c 
>>>>>>>>>> b/hw/acpi/bios-linker-loader.c
>>>>>>>>>> index d963ebe..5030cf1 100644
>>>>>>>>>> --- a/hw/acpi/bios-linker-loader.c
>>>>>>>>>> +++ b/hw/acpi/bios-linker-loader.c
>>>>>>>>>> @@ -78,6 +78,19 @@ struct BiosLinkerLoaderEntry {
>>>>>>>>>>             uint32_t length;
>>>>>>>>>>         } cksum;
>>>>>>>>>> 
>>>>>>>>>> +        /*
>>>>>>>>>> +         * COMMAND_WRITE_POINTER - write the fw_cfg file 
>>>>>>>>>> (originating from
>>>>>>>>>> +         * @dest_file) at @wr_pointer.offset, by adding a pointer 
>>>>>>>>>> to the table
>>>>>>>>>> +         * originating from @src_file. 1,2,4 or 8 byte unsigned
>>>>>>>>>> +         * addition is used depending on @wr_pointer.size.
>>>>>>>>>> +         */        
>>>>>>>> 
>>>>>>>> The words "adding" and "addition" are causing confusion here.
>>>>>>>> 
>>>>>>>> In all of the previous discussion, *addition* was out of scope from
>>>>>>>> WRITE_POINTER. Again, the firmware is specifically not required to
>>>>>>>> *read* any part of the fw_cfg blob identified by "dest_file".
>>>>>>>> 
>>>>>>>> WRITE_POINTER instructs the firmware to return the allocation address 
>>>>>>>> of
>>>>>>>> the downloaded "src_file" to QEMU. Any necessary runtime subscripting
>>>>>>>> within "src_file" is to be handled by QEMU code dynamically.
>>>>>>>> 
>>>>>>>> For example, consider that "src_file" has *several* fields that QEMU
>>>>>>>> wants to massage; in that case, indexing within QEMU code with field
>>>>>>>> offsets is simply unavoidable.      
>>>>>>> what I don't like here is that this indexing would be rather fragile
>>>>>>> and has to be done in different parts of QEMU /device, AML/.
>>>>>>> 
>>>>>>> I'd prefer this helper function to have the same @src_offset
>>>>>>> behavior as ADD_POINTER where patched address could point to
>>>>>>> any part of src_file i.e. not just beginning.      
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>>        /*
>>>>>>         * COMMAND_ADD_POINTER - patch the table (originating from
>>>>>>         * @dest_file) at @pointer.offset, by adding a pointer to the 
>>>>>> table
>>>>>>         * originating from @src_file. 1,2,4 or 8 byte unsigned
>>>>>>         * addition is used depending on @pointer.size.
>>>>>>         */
>>>>>> 
>>>>>> so the way ADD works is
>>>>>>  read at offset
>>>>>>  add table address
>>>>>>  write result at offset
>>>>>> 
>>>>>> in other words it is always beginning of table that is added.    
>>>>> more exactly it's, read at 
>>>>>  src_offset = *(dst_blob_ptr+dst_offset)
>>>>>  *(dst_blob+dst_offset) = src_blob_ptr + src_offset
>>>>> 
>>>>>> Would the following be acceptable?
>>>>>> 
>>>>>> 
>>>>>>         * COMMAND_WRITE_POINTER - update the fw_cfg file (originating 
>>>>>> from
>>>>>>         * @dest_file) at @wr_pointer.offset, by writing a pointer to the 
>>>>>> table
>>>>>>         * originating from @src_file. 1,2,4 or 8 byte unsigned value
>>>>>>         * is written depending on @wr_pointer.size.    
>>>>> it looses 'adding' part of ADD_POINTER command which handles src_offset,
>>>>> however implementing adding part looks a bit complicated
>>>>> as patched blob (dst) is not in guest memory but in QEMU and
>>>>> on reset *(dst_blob+dst_offset) should be reset to src_offset.
>>>>> Considering dst file could be device specific memory (field/blob/whatever)
>>>>> it could be hard to track/notice proper reset behavior.
>>>>> 
>>>>> So now I'm not sure if src_offset is worth adding.    
>>>> 
>>>> Right. Let's just do this math in QEMU if we have to.  
>>> Math complicates QEMU code though and not only QMEMU but AML code as well.
>>> Considering that we are adding a new command and don't have to keep
>>> any sort of compatibility we can pass src_offset as part
>>> of command instead of hiding it inside of dst_file.
>>> Something like this:
>>> 
>>>        /*
>>>         * COMMAND_WRITE_POINTER - write the fw_cfg file (originating from
>>>         * @dest_file) at @wr_pointer.offset, by writing a pointer to 
>>> @src_offset
>>>         * within the table originating from @src_file. 1,2,4 or 8 byte 
>>> unsigned
>>>         * addition is used depending on @wr_pointer.size.
>>>         */
>>>        struct {
>>>             char dest_file[BIOS_LINKER_LOADER_FILESZ];
>>>             char src_file[BIOS_LINKER_LOADER_FILESZ];
>>> -            uint32_t offset;
>>> +            uint32_t dst_offset;
>>> +            uint32_t src_offset;
>>>             uint8_t size;
>>>        } wr_pointer;  
>> 
>> 
>> As long as all users pass in 0 though there's a real possibility guests
>> will implement this incorrectly.
> We are here to ensure that at least Seabios (I'll review it)
> and OVMF (Laszlo would take care of it I suppose) do it right,
> and if there are other firmwares, they should do it correctly
> as described fix their own bugs later wrt randomly written
> implementation.
> 
>> I guess we can put in the offset just
>> behind the zero-filled padding we have there.
> I've assumed padding was there to make commands fixed size and give
> a room for future extensions so hunk changing BiosLinkerLoaderEntry
> would look like:
> 
I can’t say I follow the logic of these extra paddings.  The sizes of the 
structs are all over the place, so adding 4 bytes doesn’t do much.  I assume 
you have a good reason, though.

> diff --git a/hw/acpi/bios-linker-loader.c b/hw/acpi/bios-linker-loader.c
> index d963ebe..6983713 100644
> --- a/hw/acpi/bios-linker-loader.c
> +++ b/hw/acpi/bios-linker-loader.c
> @@ -49,6 +49,7 @@ struct BiosLinkerLoaderEntry {
>             char file[BIOS_LINKER_LOADER_FILESZ];
>             uint32_t align;
>             uint8_t zone;
> +            uint32_t padding;
I’m a little wary of doing this - in a packed structure this new field will be 
mis-aligned.
>         } alloc;
> 
>         /*
> @@ -62,6 +63,7 @@ struct BiosLinkerLoaderEntry {
>             char src_file[BIOS_LINKER_LOADER_FILESZ];
>             uint32_t offset;
>             uint8_t size;
> +            uint32_t padding;
>         } pointer;
> 
>         /*
> @@ -76,10 +78,25 @@ struct BiosLinkerLoaderEntry {
>             uint32_t offset;
>             uint32_t start;
>             uint32_t length;
> +            uint32_t padding;
>         } cksum;
> 
> +        /*
> +         * COMMAND_WRITE_POINTER - write the fw_cfg file (originating from
> +         * @dest_file) at @wr_pointer.offset, by writing a pointer to 
> @src_offset
> +         * within the table originating from @src_file. 1,2,4 or 8 byte 
> unsigned
> +         * addition is used depending on @wr_pointer.size.
> +         */
> +         struct {
> +             char dest_file[BIOS_LINKER_LOADER_FILESZ];
> +             char src_file[BIOS_LINKER_LOADER_FILESZ];
> +             uint32_t dst_offset;
> +             uint32_t src_offset;
> +             uint8_t size;
> +        } wr_pointer;
> +
>         /* padding */
> -        char pad[124];
> +        char pad[120];
wr_pointer is 121 (56 + 56 + 32 + 32 + 1), so 124 still makes sense, doesn’t 
it? (also, 124 + 4 from command) % 8 == 0, so it’s nicely aligned.
>     };
> } QEMU_PACKED;
> typedef struct BiosLinkerLoaderEntry BiosLinkerLoaderEntry;
> 
> 
>> I'm mostly concerned we are adding new features to something
>> that has been through 25 revisions already.
> It's ABI so it's worth extra effort,
> it looks like only one more revision is left and there is
> about a week left to post and merge it.
> 
>> 
>> 
>>>> 
>>>>>> 
>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>>> (1) So, the above looks correct, but please replace "adding" with
>>>>>>>> "storing", and "unsigned addition" with "store".
>>>>>>>> 
>>>>>>>> Side point: the case for ADD_POINTER is different; there we patch
>>>>>>>> several individual ACPI objects. The fact that I requested explicit
>>>>>>>> addition within the ADDR method, as opposed to pre-setting VGIA to a
>>>>>>>> nonzero offset, is an *incidental* limitation (coming from the OVMF 
>>>>>>>> ACPI
>>>>>>>> SDT header probe suppressor), and we'll likely fix that up later, with
>>>>>>>> ALLOCATE command hints or something like that. However, in
>>>>>>>> WRITE_POINTER, asking for the exact allocation address of "src_file" is
>>>>>>>> an *inherent* characteristic.
>>>>>>>> 
>>>>>>>> For reference, this is the command's description from the (not as yet
>>>>>>>> posted) OVMF series:
>>>>>>>> 
>>>>>>>> // QemuLoaderCmdWritePointer: the bytes at
>>>>>>>> // [PointerOffset..PointerOffset+PointerSize) in the writeable fw_cfg
>>>>>>>> // file PointerFile are to receive the absolute address of PointeeFile,
>>>>>>>> // as allocated and downloaded by the firmware. Store the base address
>>>>>>>> // of where PointeeFile's contents have been placed (when
>>>>>>>> // QemuLoaderCmdAllocate has been executed for PointeeFile) to this
>>>>>>>> // portion of PointerFile.
>>>>>>>> //
>>>>>>>> // This command is similar to QemuLoaderCmdAddPointer; the difference 
>>>>>>>> is
>>>>>>>> // that the "pointer to patch" does not exist in guest-physical address
>>>>>>>> // space, only in "fw_cfg file space". In addition, the "pointer to
>>>>>>>> // patch" is not initialized by QEMU with a possibly nonzero offset
>>>>>>>> // value: the base address of the memory allocated for downloading
>>>>>>>> // PointeeFile shall not increment the pointer, but overwrite it.
>>>>>>>> 
>>>>>>>> In the last SeaBIOS patch series, namely
>>>>>>>> 
>>>>>>>> [SeaBIOS] [PATCH v3 2/2] QEMU fw_cfg: Add command to write back address
>>>>>>>>                         of file
>>>>>>>> 
>>>>>>>> function romfile_loader_write_pointer() implemented just that plain
>>>>>>>> store (not an addition), and that was exactly right.
>>>>>>>> 
>>>>>>>> Continuing:
>>>>>>>> 
>>>>>>>>>> +        struct {
>>>>>>>>>> +            char dest_file[BIOS_LINKER_LOADER_FILESZ];
>>>>>>>>>> +            char src_file[BIOS_LINKER_LOADER_FILESZ];
>>>>>>>>>> +            uint32_t offset;
>>>>>>>>>> +            uint8_t size;
>>>>>>>>>> +        } wr_pointer;
>>>>>>>>>> +
>>>>>>>>>>         /* padding */
>>>>>>>>>>         char pad[124];
>>>>>>>>>>     };
>>>>>>>>>> @@ -85,9 +98,10 @@ struct BiosLinkerLoaderEntry {
>>>>>>>>>> typedef struct BiosLinkerLoaderEntry BiosLinkerLoaderEntry;
>>>>>>>>>> 
>>>>>>>>>> enum {
>>>>>>>>>> -    BIOS_LINKER_LOADER_COMMAND_ALLOCATE     = 0x1,
>>>>>>>>>> -    BIOS_LINKER_LOADER_COMMAND_ADD_POINTER  = 0x2,
>>>>>>>>>> -    BIOS_LINKER_LOADER_COMMAND_ADD_CHECKSUM = 0x3,
>>>>>>>>>> +    BIOS_LINKER_LOADER_COMMAND_ALLOCATE          = 0x1,
>>>>>>>>>> +    BIOS_LINKER_LOADER_COMMAND_ADD_POINTER       = 0x2,
>>>>>>>>>> +    BIOS_LINKER_LOADER_COMMAND_ADD_CHECKSUM      = 0x3,
>>>>>>>>>> +    BIOS_LINKER_LOADER_COMMAND_WRITE_POINTER     = 0x4,
>>>>>>>>>> };
>>>>>>>>>> 
>>>>>>>>>> enum {
>>>>>>>>>> @@ -278,3 +292,41 @@ void bios_linker_loader_add_pointer(BIOSLinker 
>>>>>>>>>> *linker,
>>>>>>>>>> 
>>>>>>>>>>     g_array_append_vals(linker->cmd_blob, &entry, sizeof entry);
>>>>>>>>>> }
>>>>>>>>>> +
>>>>>>>>>> +/*
>>>>>>>>>> + * bios_linker_loader_write_pointer: ask guest to write a pointer 
>>>>>>>>>> to the
>>>>>>>>>> + * source file into the destination file, and write it back to QEMU 
>>>>>>>>>> via
>>>>>>>>>> + * fw_cfg DMA.
>>>>>>>>>> + *
>>>>>>>>>> + * @linker: linker object instance
>>>>>>>>>> + * @dest_file: destination file that must be written
>>>>>>>>>> + * @dst_patched_offset: location within destination file blob to be 
>>>>>>>>>> patched
>>>>>>>>>> + *                      with the pointer to @src_file, in bytes
>>>>>>>>>> + * @dst_patched_offset_size: size of the pointer to be patched
>>>>>>>>>> + *                      at @dst_patched_offset in @dest_file blob, 
>>>>>>>>>> in bytes
>>>>>>>>>> + * @src_file: source file who's address must be taken
>>>>>>>>>> + */
>>>>>>>>>> +void bios_linker_loader_write_pointer(BIOSLinker *linker,
>>>>>>>>>> +                                    const char *dest_file,
>>>>>>>>>> +                                    uint32_t dst_patched_offset,
>>>>>>>>>> +                                    uint8_t dst_patched_size,
>>>>>>>>>> +                                    const char *src_file)        
>>>>>>>>> API is missing "src_offset" even though it's not used in this series,
>>>>>>>>> a patch on top to fix it up is ok for me as far as Seabios/OVMF
>>>>>>>>> counterpart can handle src_offset correctly from starters.        
>>>>>>>> 
>>>>>>>> According to the above, it is the right thing not to add "src_offset"
>>>>>>>> here. The documentation on the command is slightly incorrect (and 
>>>>>>>> causes
>>>>>>>> confusion), but the helper function's signature and comments are okay.
>>>>>>>> 
>>>>>>>>> 
>>>>>>>>>> +{
>>>>>>>>>> +    BiosLinkerLoaderEntry entry;
>>>>>>>>>> +    const BiosLinkerFileEntry *source_file =
>>>>>>>>>> +        bios_linker_find_file(linker, src_file);
>>>>>>>>>> +
>>>>>>>>>> +    assert(source_file);        
>>>>>>>> 
>>>>>>>> I wish we kept the following asserts from 
>>>>>>>> bios_linker_loader_add_pointer():
>>>>>>>> 
>>>>>>>>    assert(dst_patched_offset < dst_file->blob->len);
>>>>>>>>    assert(dst_patched_offset + dst_patched_size <= 
>>>>>>>> dst_file->blob->len);
>>>>>>>> 
>>>>>>>> Namely, just because the dst_file is never supposed to be downloaded by
>>>>>>>> the firmware, it still remains a requirement that the "dst file offset
>>>>>>>> range" that is to be rewritten *do fall* within the dst file.
>>>>>>>> 
>>>>>>>> Nonetheless, this is not critical. (OVMF at least verifies it anyway.)
>>>>>>>> 
>>>>>>>> Summary (from my side anyway): I feel that the documentation of the new
>>>>>>>> command is very important. Please fix it up as suggested under (1), in
>>>>>>>> v7. Regarding the asserts, I'll let you decide.
>>>>>>>> 
>>>>>>>> With the documentation fixed up:
>>>>>>>> 
>>>>>>>> Reviewed-by: Laszlo Ersek <address@hidden>
>>>>>>>> 
>>>>>>>> (If you don't wish to post a v7, I'm also completely fine if Michael or
>>>>>>>> someone else fixes up the docs as proposed in (1), before committing 
>>>>>>>> the
>>>>>>>> patch.)
>>>>>>>> 
>>>>>>>> Thanks!
>>>>>>>> Laszlo
>>>>>>>> 
>>>>>>>>>> +    memset(&entry, 0, sizeof entry);
>>>>>>>>>> +    strncpy(entry.wr_pointer.dest_file, dest_file,
>>>>>>>>>> +            sizeof entry.wr_pointer.dest_file - 1);
>>>>>>>>>> +    strncpy(entry.wr_pointer.src_file, src_file,
>>>>>>>>>> +            sizeof entry.wr_pointer.src_file - 1);
>>>>>>>>>> +    entry.command = 
>>>>>>>>>> cpu_to_le32(BIOS_LINKER_LOADER_COMMAND_WRITE_POINTER);
>>>>>>>>>> +    entry.wr_pointer.offset = cpu_to_le32(dst_patched_offset);
>>>>>>>>>> +    entry.wr_pointer.size = dst_patched_size;
>>>>>>>>>> +    assert(dst_patched_size == 1 || dst_patched_size == 2 ||
>>>>>>>>>> +           dst_patched_size == 4 || dst_patched_size == 8);
>>>>>>>>>> +
>>>>>>>>>> +    g_array_append_vals(linker->cmd_blob, &entry, sizeof entry);
>>>>>>>>>> +}
>>>>>>>>>> diff --git a/include/hw/acpi/bios-linker-loader.h 
>>>>>>>>>> b/include/hw/acpi/bios-linker-loader.h
>>>>>>>>>> index fa1e5d1..f9ba5d6 100644
>>>>>>>>>> --- a/include/hw/acpi/bios-linker-loader.h
>>>>>>>>>> +++ b/include/hw/acpi/bios-linker-loader.h
>>>>>>>>>> @@ -26,5 +26,11 @@ void bios_linker_loader_add_pointer(BIOSLinker 
>>>>>>>>>> *linker,
>>>>>>>>>>                                     const char *src_file,
>>>>>>>>>>                                     uint32_t src_offset);
>>>>>>>>>> 
>>>>>>>>>> +void bios_linker_loader_write_pointer(BIOSLinker *linker,
>>>>>>>>>> +                                      const char *dest_file,
>>>>>>>>>> +                                      uint32_t dst_patched_offset,
>>>>>>>>>> +                                      uint8_t dst_patched_size,
>>>>>>>>>> +                                      const char *src_file);
>>>>>>>>>> +
>>>>>>>>>> void bios_linker_loader_cleanup(BIOSLinker *linker);
>>>>>>>>>> #endif

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