Re: [PATCH v2] tools/hvmloader: implement Intel IGD extended VBT support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.08.2026 14:36, Chuck Zmudzinski wrote:
> On 8/19/2026 8:16 AM, Chuck Zmudzinski wrote:
>> Yes, I agree that v3 of the patch to hvmloader should presume that the DM 
>> can get
>> a copy of the OpRegion and read its contents so most of this can be done in 
>> the
>> DM instead of in hvmloader. So from hvmloader's perspective, the patch will 
>> be more
>> about avoiding the layering violation than anything else.
> 
> However, there is one advantage, from the viewpoint of the Xen virtualization 
> platform
> as a whole, to do the patching of the OpRegion in hvmloader instead of in the 
> DM.
> 
> If we patch the OpRegion in hvmloader as v2 of this patch does, we provide a 
> common
> solution for extended VBT support for Intel IGD devices that would be 
> compatible with
> all DM implementations, not just with Qemu. So why not do the patching of the 
> OpRegion
> in hvmloader?

As indicated before: If the OpRegion holds data that is needed to drive the
device, and if the OpRegion is exposed writable to guests, then guest can
screw up that data such that subsequent guests won't work anymore. Hence
exposing to guests (which includes hvmloader) needs to be stopped, or at
least be limited to r/o. That, in fact, includes exposing to any privilege-
restricted DM as well.

Exposing r/o may be entirely okay (i.e. may not be a layering violation),
depending how exactly an OpRegion surfaces for a device (on the host). Aiui
it's not addressed by any of the BARs, yet it looks like it needs similar
treatment. Earlier on we also talked about the region not necessarily being
page-aligned. That poses, even with r/o exposure, the question of other
data on the same (leading / trailing) pages. This may imply that the
copying needs to be done strictly in Dom0, for both DM and guest to only
ever act on copies (which may then as well be r/w).

Jan