Re: [PATCH] vpci: Add resizable bar support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.11.2024 11:30, Roger Pau Monné wrote:
> On Wed, Nov 13, 2024 at 10:00:33AM +0000, Chen, Jiqian wrote:
>> On 2024/11/13 17:30, Roger Pau Monné wrote:
>>> On Wed, Nov 13, 2024 at 04:00:27PM +0800, Jiqian Chen wrote:
>>>> Some devices, like discrete GPU of amd, support resizable bar capability,
>>>> but vpci of Xen doesn't support this feature, so they fail to resize bars
>>>> and then cause probing failure.
>>>>
>>>> According to PCIe spec, each bar that support resizing has two registers,
>>>> PCI_REBAR_CAP and PCI_REBAR_CTRL, so add these two registers and their
>>>> corresponding handler into vpci.
>>>>
>>>> PCI_REBAR_CAP is RO, only provide reading.
>>>>
>>>> PCI_REBAR_CTRL only has bar size is RW, so add write function to support
>>>> setting the new size.
>>>
>>> I think the logic to handle resizable BAR could be much simpler.  Some
>>> time ago I've made a patch to add support for it, but due to lack of
>>> hardware on my side to test it I've never submitted it.
>>>
>>> My approach would be to detect the presence of the
>>> PCI_EXT_CAP_ID_REBAR capability in init_header(), and if the
>>> capability is present force the sizing of BARs each time they are
>>> mapped in modify_bars().  I don't think we need to trap accesses to
>>> the capability itself, as resizing can only happen when memory
>>> decoding is not enabled for the device.  It's enough to fetch the size
>>> of the BARs ahead of each enabling of memory decoding.
>>>
>>> Note that memory decoding implies mapping the BARs into the p2m, which
>>> is already an expensive operation, the extra sizing is unlikely to
>>> make much of a difference performance wise.
>>>
>>> I've found the following on my git tree and rebased on top of staging:
>> OK.
>> Do you need me to validate your patch in my environment?
> 
> Yes please, I have no way to test it.  Let's see what others think
> about the different approaches.

I'd certainly prefer your simpler form, if it's safe and fits the needs.

>> And I have one question: where does your patch do writing the resizing size 
>> into hardware?
> 
> dom0 has unrestricted access to the resize capability, so the value
> written by dom0 is propagated to the hardware without modification.
> 
> I would be wary of exposing the resize capability to untrusted
> domains, as allowing a domU to change the size of BARs can lead to
> overlapping if the hardware domain hasn't accounted for the increase
> in BAR size.

Question is how the feature is used in practice: If it was a driver to
request the re-size, I'd have a hard time seeing how we could make that
work without intercepting accesses to the capability for DomU-s (implying
to expose it in the first place, of course).

Jan