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

[Igor Mammedov]

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.

> 
> 
> > 
> >   
> > > (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    
> > > >     
> > >