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

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

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.

> 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?

I'm happy to just use cache_flush_permitted() which is likely more
correct given the context here.

Thanks, Roger.