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
