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

[Noboru Iwamatsu <n_iwamatsu@xxxxxxxxxxxxxx>]

Hi,

Generally, we should validate the checksum right.

Weidong, have you seen the checksum adjusting?

I only saw it in Q35+IGD.
You know my Q35 is buggy. If this is the only issue of my Q35,
2nd patch (validate and disable) is the best way, I think.

Or, is this a IGD specific issue?
If so, checksum adjusting should be a workaround for IGD pass-through.
I assume IGD's VGABIOS is included in the system BIOS and might be
expanded to the RAM in a different way from the PCI gfx card.

I attach 3rd patch:
When invalid checksum is detected, adjust it if igd_passthru is enabled. 
In other case, stop loading.
I tried this on my Q35+IGD, the checksum has been adjusted and worked.

Do you have any idea?

Regards,
Noboru.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@xxxxxxxxxxxxxx>

> Noboru Iwamatsu wrote:
> > 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.
> Yes, I ack this one. Acked-by: Weidong Han <weidong.han@xxxxxxxxx>
>
> Regards,
> Weidong
> > Noboru.
> >
> > Signed-off-by: Noboru Iwamatsu <n_iwamatsu@xxxxxxxxxxxxxx>

Attachment: adjust-checksum-for-igd.patch
Description: Text document

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