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

[Chuck Zmudzinski <brchuckz@xxxxxxx>]

On 8/18/2026 3:17 AM, Jan Beulich wrote:
> On 17.08.2026 18:04, Chuck Zmudzinski wrote:
>> On 8/17/2026 4:42 AM, Jan Beulich wrote:
>>> On 14.08.2026 17:23, Chuck Zmudzinski wrote:
>>>> On 8/14/2026 9:46 AM, Jan Beulich wrote:
>>>>> On 14.08.2026 15:18, Chuck Zmudzinski wrote:
>>>>>> On 8/14/2026 3:35 AM, Jan Beulich wrote:
>>>>>>> On 14.08.2026 02:45, Chuck Zmudzinski wrote:
>>>>>>>> On 8/13/2026 6:35 AM, Jan Beulich wrote:
>>>>>>>>> On 02.08.2026 07:08, Chuck Zmudzinski wrote:
>>>>>>>>>> -- snip --
>>>>>>>>>> +    /*
>>>>>>>>>> +     * Write rvda_host as 2 successive 32-bit values
>>>>>>>>>> +     * to communicate location of the VBT to the device
>>>>>>>>>> +     * model. If rvda_host is not 0, The device model
>>>>>>>>>> +     * unmaps the OpRegion and eventually maps the VBT
>>>>>>>>>> +     * after we also write the guest address where the
>>>>>>>>>> +     * VBT will be mapped.
>>>>>>>>>> +     *
>>>>>>>>>> +     * If we send rvda_host = 0 to the device model, it
>>>>>>>>>> +     * will assume we do not need OpRegion 2 support and
>>>>>>>>>> +     * it will not unmap the OpRegion.
>>>>>>>>>> +     */
>>>>>>>>>> +    pci_writel(vga_devfn, PCI_INTEL_OPREGION,
>>>>>>>>>> +               (uint32_t)(rvda_host & 0xfffffffful));
>>>>>>>>>> +    unsigned long rvda_host_upper_32 = (uint64_t)rvda_host >> 32;
>>>>>>>>>> +    pci_writel(vga_devfn, PCI_INTEL_OPREGION,
>>>>>>>>>> +               (uint32_t)rvda_host_upper_32);
>>>>>>>>>
>>>>>>>>> Why would you need to communicate a host property to the DM?
>>>>>>>>
>>>>>>>> The DM cannot access the host rvda value because it is only accessible
>>>>>>>> from the host kernel, and the DM is only a user-space process on the 
>>>>>>>> host.
>>>>>>>
>>>>>>> I don't follow this: Anything the guest can access should also be 
>>>>>>> accessible
>>>>>>> by its DM.
>>>>>>
>>>>>> I think the host OpRegion is not currently accessible by the DM.
>>>>>
>>>>> Can you explain to me how the region becomes accessible to the guest?
>>>>> That would then (hopefully) help me understand why the DM would not have
>>>>> access. Fundamentally any MMIO and any I/O ports that are assigned to a
>>>>> guest are also assigned to its DM.
>>>>
>>>> Currently, in the device model (Qemu) we have:
>>>>
>>>>     ret = xc_domain_memory_mapping(xen_xc, xen_domid,
>>>>             (unsigned long)(igd_guest_opregion >> XC_PAGE_SHIFT),
>>>>             (unsigned long)(igd_host_opregion >> XC_PAGE_SHIFT),
>>>>             XEN_PCI_INTEL_OPREGION_PAGES,
>>>>             DPCI_ADD_MAPPING);
>>>>
>>>> That statement is in the igd_write_opregion(...) function in the
>>>> hw/xen/xen_pt_graphics.c file of the upstream Qemu source.
>>>>
>>>> If I understand our current implementation correctly, this statement
>>>> is what gives the guest access to the host OpRegion (3 pages as defined
>>>> by XEN_PCI_INTEL_OPREGION_PAGES, and in agreement with IGD_OPREGION_PAGES
>>>> in hvmloader code).
>>>
>>> No, it introduces mappings of those pages into the guest's P2M.
>>>
>>>> I don't think this statement makes the host OpRegion
>>>> accessible to the device model, though, so I think, if I understand your
>>>> comment in an earlier about my patch resulting in what you called a 
>>>> "layering
>>>> violation" correctly, that our current implementation is also guilty of 
>>>> this
>>>> same kind of "layering violation."
>>>
>>> That code, if it can be successfully executed, indeed doesn't grant any
>>> permissions (to the DM or the guest). Instead it proves that the DM has the
>>> needed permissions to access the pages itself.
>> 
>> So, are you saying it should be possible, without any patches to either Xen 
>> or
>> the Linux kernel, for Qemu to get a pointer to the OpRegion? If so, how?
>> 
>> I think I could implement what you proposed in an earlier message and do
>> all (or most) of this in the DM instead of here in hvmloader:
>> 
>>> The more correct thing to do might be for the DM to
>>> put in place a copy before the guest (i.e. hvmloader) even gains control.
>>> (How in turn the DM would learn of the contents of the opregion is a
>>> separate question then.)
>> 
>> Actually, when I was developing this patch, I tried first to do it that
>> way, but the problem was, I could not find a way to get a pointer to the
>> host OpRegion in Qemu.
>> 
>> So, how can I get a pointer to the host OpRegion in Qemu?
> 
> You don't ask me this question, do you?

Are you offended I asked this question? If so, I am sorry. You make me
afraid to ask it again so I will not do so unless you permit to do so
again.

All I can say is that surely qemu
> has an existing way to map (host) physical memory; see e.g. how
> xen_pt_msix_init() (imo bogusly) maps the physical MSI-X table of a
> device. "Bogusly" there because that's another layering violation. Plus
> (independently) there and here there's the issue of how to accomplish
> things when not running in Dom0, or when running de-privileged in Dom0.

Well, that only proves Qemu *might* be able to access the MSI-X table of
a device, that is, if the calls to open /dev/mem and mmap it succeed.
Why is the MSI-X table all of the sudden relevant? Even if Qemu
can access the MSI-X table of some device, that does not prove that
Qemu can access the host OpRegion of an Intel IGD. So I think my point
still stands: I still don't see proof that it is possible for Qemu
to get a pointer to the host OpRegion without any patches to the current
implementations of Xen and the Linux kernel.

Perhaps I should accept your indications that it must be possible to get a
pointer to the host OpRegion. I will admit maybe it is possible and I have
not yet found out how to do it, but I have hardware I can experiment
with, and for me, that is the final authority. Proof for me only comes
from my own tests and experiments that I conduct on my hardware. Until I
see how I can get a pointer to the OpRegion in Qemu on my hardware and
actually realize that goal, I remain skeptical that it is possible to do
so solely by patching Qemu and not patching either Xen or the Linux kernel.

Chuck 

> 
> Jan