Re: [PATCH 8/9] xen: introduce flag when a domain requires cache control

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

On Mon, May 12, 2025 at 05:24:01PM +0200, Jan Beulich wrote:
> On 06.05.2025 10:31, Roger Pau Monne wrote:
> > Such flag is added to the domain create hypercall, and a matching option is
> > added to xl and libxl to set the flag: `cache_control`.  When the flag is
> > set, the domain is allowed the usage of cache control operations.  If the
> > flag is not explicitly set, libxl will set it if the domain has any `iomem`
> > or `ioports` ranges assigned.
> > 
> > Modify cache_flush_permitted() helper so that it's return value is no
> > longer based on the IO resources assigned to the domain, but rather whether
> > the domain has been explicitly allowed control over the cache, or if it has
> > IOMMU support and there's a single IOMMU in the system that doesn't allow
> > forcing snooping behind the guest back.  As a result of this, the return of
> > cache_flush_permitted() is constant for the lifetime of a domain, based on
> > the domain creation flags and the capabilities of the IOMMU if enabled
> > for the domain.
> 
> This then further emphasizes the remark made for patch 7.

Hm, I think you are referring to the remark about how PCI device
without IO resources would be handled then, and what would
cache_flush_permitted() return then?

IMO the benefit of the approach presented here is that it aims to know
whether a domain needs cache control to be set at creation time, and
not altered during it's runtime.

Thanks, Roger.