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

[Jan Beulich <jbeulich@suse.com>]

On 18.08.2026 13:52, Chuck Zmudzinski wrote:
> 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.

"Offended" is the wrong word; "very puzzled" may better get it. I'm not a
qemu person, and I never have been. I can't really help much there.

> 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.

The MSI-X table (and it being accessible to qemu) is the best analogy I
could come up with, as that's one tiny area of qemu that I know at least
a little.

From a Xen perspective, this analogy should be sufficient: All you need
from Xen is for it to permit to establish mappings of the underlying page.
As I've pointed out when commenting on a code fragment you presented, the
DM (domain) looks to have permission. Everything else is a matter of
establishing such a mapping. There the MSI-X table code may also guide
you. (Sadly it may also misguide you, since (a) I don't know whether it's
appropriate to do things this way in qemu, and since (b) it is, as said,
imo a layering violation.)

Jan