Re: [Qemu-devel] [PATCH] cpu_physical_memory_write_rom() needs to do TB invalidates

[Alexander Graf]

On 22.08.2012, at 13:38, David Gibson wrote:

> On Wed, Aug 22, 2012 at 09:05:52AM +0200, Jan Kiszka wrote:
>> On 2012-08-22 08:47, Jan Kiszka wrote:
>>> On 2012-08-22 07:57, David Gibson wrote:
>>>> On Wed, Aug 22, 2012 at 07:55:31AM +0200, Alexander Graf wrote:
>>>>> 
>>>>> On 22.08.2012, at 06:59, David Gibson wrote:
>>>>> 
>>>>>> cpu_physical_memory_write_rom(), despite the name, can also be used to
>>>>>> write images into RAM - and will often be used that way if the machine
>>>>>> uses load_image_targphys() into RAM addresses.
>>>>>> 
>>>>>> However, cpu_physical_memory_write_rom(), unlike cpu_physical_memory_rw()
>>>>>> does invalidate any cached TBs which might be affected by the region
>>>>>> written.
>>>>>> 
>>>>>> This was breaking reset (under full emu) on the pseries machine - we 
>>>>>> loaded
>>>>>> our firmware image into RAM, and while executing it rewrite the code at
>>>>>> the entry point (correctly causing a TB invalidate/refresh).  When we
>>>>>> reset the firmware image was reloaded, but the TB from the rewrite was
>>>>>> still active and caused us to get an illegal instruction trap.
>>>>>> 
>>>>>> This patch fixes the bug by duplicating the tb invalidate code from
>>>>>> cpu_physical_memory_rw() in cpu_physical_memory_write_rom().
>>>>>> 
>>>>>> Signed-off-by: David Gibson <address@hidden>
>>>>>> ---
>>>>>> exec.c |    7 +++++++
>>>>>> 1 file changed, 7 insertions(+)
>>>>>> 
>>>>>> diff --git a/exec.c b/exec.c
>>>>>> index 5834766..eff40d7 100644
>>>>>> --- a/exec.c
>>>>>> +++ b/exec.c
>>>>>> @@ -3523,6 +3523,13 @@ void 
>>>>>> cpu_physical_memory_write_rom(target_phys_addr_t addr,
>>>>>>            /* ROM/RAM case */
>>>>>>            ptr = qemu_get_ram_ptr(addr1);
>>>>>>            memcpy(ptr, buf, l);
>>>>>> +            if (!cpu_physical_memory_is_dirty(addr1)) {
>>>>>> +                /* invalidate code */
>>>>>> +                tb_invalidate_phys_page_range(addr1, addr1 + l, 0);
>>>>>> +                /* set dirty bit */
>>>>>> +                cpu_physical_memory_set_dirty_flags(
>>>>>> +                    addr1, (0xff & ~CODE_DIRTY_FLAG));
>>>>>> +            }
>>>>> 
>>>>> Can't we just call cpu_physical_memory_rw in the RAM case? The
>>>>> function only tries to not do MMIO accesses on ROM pages, right?
>>>> 
>>>> Maybe.  It's not clear at all to me what cases
>>>> cpu_physical_memory_write_rom() is supposed to be for, as opposed to
>>>> just using cpu_physical_memory_rw().
>>> 
>>> write_rom ignores write protection - that you usually find on ROMs. That
>>> makes no difference under KVM so far as there we lack read-only
>>> sections. But that will be fixed soon, patches are on the list.
>> 
>> In fact, it does make a difference also for KVM mode as
>> cpu_physical_memory_rw works from userspace while the limitation only
>> affects guest code running under KVM control.
> 
> Ok, so IIUC, that means we do need the cpu_physical_memory_write_rom()
> version for load_image_targphys(), and so my original patch is
> basically the right fix.

Sure it is, I don't think anyone argued about that :). But it's duplicating 
code in a slow path. So my proposal was instead of doing the write manually in 
the "this is read-write RAM" case, just fall back to the known-to-work 
cpu_physical_memory_rw for those pages. That would make the rom function 
smaller, more obvious and duplicate less code.


Alex