Re: [PATCH 7/9] xen/x86: rename cache_flush_permitted() to has_arch_io_resources()

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

On Fri, May 16, 2025 at 09:07:43AM +0200, Jan Beulich wrote:
> On 15.05.2025 12:28, Roger Pau Monné wrote:
> > On Mon, May 12, 2025 at 05:16:02PM +0200, Jan Beulich wrote:
> >> On 06.05.2025 10:31, Roger Pau Monne wrote:
> >>> To better describe the underlying implementation.  Define
> >>> cache_flush_permitted() as an alias of has_arch_io_resources(), so that
> >>> current users of cache_flush_permitted() are not effectively modified.
> >>>
> >>> With the introduction of the new handler, change some of the call sites of
> >>> cache_flush_permitted() to instead use has_arch_io_resources() as such
> >>> callers are not after whether cache flush is enabled, but rather whether
> >>> the domain has any IO resources assigned.
> >>>
> >>> Take the opportunity to adjust l1_disallow_mask() to use the newly
> >>> introduced has_arch_io_resources() macro.
> >>
> >> While I'm happy with everything else here, to me it's at least on the
> >> edge whether cache_flush_permitted() wouldn't be the better predicate
> >> to use there, for this being about ...
> >>
> >>> --- a/xen/arch/x86/mm.c
> >>> +++ b/xen/arch/x86/mm.c
> >>> @@ -172,8 +172,7 @@ static DEFINE_SPINLOCK(subpage_ro_lock);
> >>>  
> >>>  #define l1_disallow_mask(d)                                     \
> >>>      (((d) != dom_io) &&                                         \
> >>> -     (rangeset_is_empty((d)->iomem_caps) &&                     \
> >>> -      rangeset_is_empty((d)->arch.ioport_caps) &&               \
> >>> +     (!has_arch_io_resources(d) &&                              \
> >>>        !has_arch_pdevs(d) &&                                     \
> >>>        is_pv_domain(d)) ?                                        \
> >>>       L1_DISALLOW_MASK : (L1_DISALLOW_MASK & ~PAGE_CACHE_ATTRS))
> >>
> >> ... cachability, which goes hand in hand with the ability to also
> >> flush cache contents.
> > 
> > Hm, I was on the edge here, in fact I've previously coded this using
> > cache_flush_permitted(), just to the change back to
> > has_arch_io_resources().  If you think cache_flush_permitted() is
> > better I'm fine with that.
> 
> I think that would be better here, yet as you say - it's not entirely
> clear cut either way.

I've reverted this chunk of the change and left the code as-is for the
time being.

> >> Tangentially - is it plausible for has_arch_io_resources() to return
> >> false when has_arch_pdevs() returns true? Perhaps there are exotic
> >> PCI devices (but non-bridges) which work with no BARs at all ...
> > 
> > I guess it's technically possible, albeit very unlikely?  How would
> > the OS interact with such device then, exclusively with PCI config
> > space accesses?
> 
> Yes, that's what I'd expect for such devices. Looking around, there
> are numerous such devices (leaving aside bridges). Just that it looks
> implausible to me that one would want to pass those through to a guest.

Well, we also need to consider dom0 here (either PV or PVH), which
will get those devices passed through.  I assume those are mostly
system devices, and hence there's usually no interaction of the OS
with them.

I'm thinking that our definition of cache_flush_permitted() is not
fully accurate then, we would need to also account for any PCI devices
being assigned to the guest, even if those have no IO resources?

Thanks, Roger.