Re: [PATCH] vpci: Add resizable bar support

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

> 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.

Thanks, Roger.