Re: Re: [PATCH][RFC] gfx_passthru: warning when vgabios rom has invalid checksum

[Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>]

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Hi Weidong,

 > How did you recalculate it? is it possible to recalculate it when detect
 > invalid checksum?

Checking and recalculating the VGABIOS checksum is being processed
in setup_vga_pt() in hw/pass-through.c.
I thought you have written that part.

If the checksum is invalid, current setup_vga_pt() always overwrites
it with the recalculated value.

What I mention is: the VGABIOS checksum must have been already
recalculated by system BIOS. So, when setup_vga_pt() detects the
invalid one, something might be wrong.

But, I'm not sure all BIOS should work as described in PCIFW Spec.
Actually, my Q35 had invalid checksum.


I attach another patch:
This stops loading vgabios if the checksum is wrong.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>

> Noboru Iwamatsu wrote:
>> Hi,
>>
>> >> Intel DX58SO + GeForce GTS250: checksum is OK.
>> >> Intel DX58SO + GeForce 9600GT: checksum is OK.
>> >> Fujitsu Q35 M/B + IGD: checksum is bad.
>> >>
>> > What does "checksum is bad" mean here?
>>
>> It means shadowed VGABIOS has invalid checksum
>> and requires recalculating.
>>
>> >> What do you think, Weidong?
>> >> Is just warning enough? Or, should we stop loading the rom?
>> >>
>> >>
>> > Does it still work or not when checksum is bad? If it works, we should
>> > not stop loading the rom.
>>
>> When the checksum is bad,
>> if it is not recalculated, guest has some trouble about handling the
>> VGA (unable to show BIOS screen, unable to load the gfx driver
>> properly, ...).
> For this case, it's reasonable to warning and stop loading rom. We
> didn't encounter this issue.
>> If it is recalculated, it seems to work fine.
> How did you recalculate it? is it possible to recalculate it when detect
> invalid checksum?
>
> Regards,
> Weidong
>
>> Regards,
>> Noboru.
>>
>>> Noboru Iwamatsu wrote:
>>>> Hi,
>>>>
>>>> According to the "PCI Firmware Spec Rev 3.0",
>>>> the system firmware have to write the new checksum after resizing
>>>> the expansion ROM area.
>>>>
>>>> So, when re-using the shadowed VGABIOS, the checksum must
>>>> be valid, and if it is invalid, we should consider the BIOS has
>>>> a bug or memory area is corrupted.
>>>>
>>>> This patch just add the warning message when checksum requires
>>>> recalculation.
>>>>
>>>> I tried this following environments.
>>>>
>>>> Intel DX58SO + GeForce GTS250: checksum is OK.
>>>> Intel DX58SO + GeForce 9600GT: checksum is OK.
>>>> Fujitsu Q35 M/B + IGD: checksum is bad.
>>>>
>>> What does "checksum is bad" mean here?
>>>> What do you think, Weidong?
>>>> Is just warning enough? Or, should we stop loading the rom?
>>>>
>>>>
>>> Does it still work or not when checksum is bad? If it works, we should
>>> not stop loading the rom.
>>>
>>> Regards,
>>> Weidong
>>>> Regards,
>>>> Noboru.
>>>>
>>
>>
>
>
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[-- Attachment #2: vgabios-checksum-validation.patch --]
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diff --git a/hw/pass-through.c b/hw/pass-through.c
index ecb3d6f..8775956 100644
--- a/hw/pass-through.c
+++ b/hw/pass-through.c
@@ -4258,11 +4258,15 @@ static int setup_vga_pt(void)
         goto out;
     }
 
-    /* Adjust the bios checksum */
+    /* Validate the bios checksum */
     for ( c = (char*)bios; c < ((char*)bios + bios_size); c++ )
         checksum += *c;
     if ( checksum )
-        bios[bios_size - 1] -= checksum;
+    {
+        PT_LOG("vga bios checksum is invalid!\n");
+        rc = -1;
+        goto out;
+    }
 
     cpu_physical_memory_rw(0xc0000, bios, bios_size, 1);
 

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