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

[Jan Beulich <jbeulich@xxxxxxxx>]

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? 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.

Jan