Re: [PATCH v1 5/5] vpci: allow 32-bit BAR writes with memory decoding enabled

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

On Thu, Jun 05, 2025 at 12:41:06PM +0200, Jan Beulich wrote:
> On 31.05.2025 14:54, Stewart Hildebrand wrote:
> > Currently, Xen vPCI refuses BAR writes if the BAR is mapped in p2m. If
> > firmware initializes a 32-bit BAR to a bad address, Linux may try to
> > write a new address to the BAR without disabling memory decoding. Since
> > Xen refuses such writes, the BAR (and thus PCI device) will be
> > non-functional.
> 
> Doing this for 32-bit BARs only, with not even an outline what to do about
> the same issue with 64-bit ones, seems like it won't buy us very much.

IIRC Linux will disable decoding in the common case when updating the
position of a 64bit BAR.  However it won't disable decoding for 32bit
BARs.  I think that's why Stewart cares more about the 32bit case than
the 64bit one.

Regards, Roger.