Re: [PATCH v4] vpci/msix: handle accesses adjacent to the MSI-X table

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

On Wed, Mar 29, 2023 at 09:55:27AM +0200, Jan Beulich wrote:
> On 24.03.2023 13:17, Roger Pau Monne wrote:
> > The handling of the MSI-X table accesses by Xen requires that any
> > pages part of the MSI-X related tables are not mapped into the domain
> > physmap.  As a result, any device registers in the same pages as the
> > start or the end of the MSIX or PBA tables is not currently
> > accessible, as the accesses are just dropped.
> > 
> > Note the spec forbids such placing of registers, as the MSIX and PBA
> > tables must be 4K isolated from any other registers:
> > 
> > "If a Base Address register that maps address space for the MSI-X
> > Table or MSI-X PBA also maps other usable address space that is not
> > associated with MSI-X structures, locations (e.g., for CSRs) used in
> > the other address space must not share any naturally aligned 4-KB
> > address range with one where either MSI-X structure resides."
> > 
> > Yet the 'Intel Wi-Fi 6 AX201' device on one of my boxes has registers
> > in the same page as the MSIX tables, and thus won't work on a PVH dom0
> > without this fix.
> > 
> > In order to cope with the behavior passthrough any accesses that fall
> > on the same page as the MSIX tables (but don't fall in between) to the
> > underlying hardware.  Such forwarding also takes care of the PBA
> > accesses, so it allows to remove the code doing this handling in
> > msix_{read,write}.  Note that as a result accesses to the PBA array
> > are no longer limited to 4 and 8 byte sizes, there's no access size
> > restriction for PBA accesses documented in the specification.
> 
> I should have (re)checked the spec. There is a restriction, common to table
> and PBA: "For all accesses to MSI-X Table and MSI-X PBA fields, software
> must use aligned full DWORD or aligned full QWORD transactions; otherwise,
> the result is undefined."

Thanks for noticing, I really wasn't able to spot it when I checked.
I guess because of the intermixed "Implementation Note" sections in
the spec.

Will amend and resend v5 then.

Roger.