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

[Jan Beulich <jbeulich@xxxxxxxx>]

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."

Jan