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 Han writes ("Re: [Xen-devel] Re: [PATCH][RFC] gfx_passthru: warning when vgabios rom has invalid checksum"):
>> Now I understand. Because your Q35 works with recalculated checksum, I
>> prefer to only add a warning message, and continue to load rom for gfx
>> passthru.
>
> Having read this thread I'm still a bit confused.  The problem is that
> the VGA BIOS on the graphics card is broken and has a broken checksum,
> and the proposed workaround is to recalculate the checksum for the
> benefit of the guest ?

In the native environment, the VGABIOS, the expansion ROM on the
graphics card, is placed into the 0C0000h address space, and then
executed. Of course, the checksum of the ROM must be valid.

After this initialization, the system BIOS, the actual BIOS of the M/B,
can resize the expansion ROM code to reduce the amount of occupied 
space. If the system BIOS resizes it, a new checksum must be calculated
and stored in the ROM image that is on the RAM.

So, normally, shadowed VGABIOS, that is placed in 0C0000h, is already
modified and its checksum must be recalculated.

Qemu-dm copies 0C0000h's contents of the dom0 to guest's 0C0000h.
Guest re-uses dom0's used-up VGABIOS.

The problem that I mentioned is about this recalculated checksum.

System BIOS must guarantee the checksum after the resizing, but,
some M/B does not.
However, after adjusting the checksum, guest seems to work, and
current qemu-dm does so. The buggy system BIOS might just forgets
to recalculate.

Should we check strictly here?

> Does this incorrectly checksummed BIOS work natively (ie without
> passthrough) and if so why is passthrough different ?  Alternatively
> if it doesn't work native why are we trying to make it work with
> passthrough ?
>
> On another level, Weidong, are you suggesting you'd like to see Noboru
> produce a different patch which just produces a warning ?

I sent "just warning" patch on the first of this thread.
I resend it.

Noboru.

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



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diff --git a/hw/pass-through.c b/hw/pass-through.c
index ecb3d6f..fadd358 100644
--- a/hw/pass-through.c
+++ b/hw/pass-through.c
@@ -4262,7 +4262,10 @@ static int setup_vga_pt(void)
     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 adjusted!\n");
+    }
 
     cpu_physical_memory_rw(0xc0000, bios, bios_size, 1);
 

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