Hi Andrew,

On Fri, 8 Oct 2021, 20:07 Andrew Cooper, <andrew.cooper3@citrix.com> wrote:

> On 06/10/2021 18:40, Rahul Singh wrote:
> > Introduce XEN_DOMCTL_CDF_vpci flag to enable VPCI support in XEN.
> > Reject the use of this new flag for x86 as VPCI is not supported for
> > DOMU guests for x86.
> >
> > Signed-off-by: Rahul Singh <rahul.singh@arm.com>
> > Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
> > Acked-by: Christian Lindig <christian.lindig@citrix.com>
>
> I'm afraid this logic is broken.
>
> There's no matching feature to indicate to the toolstack whether
> XEN_DOMCTL_CDF_vpci will be accepted or not.  The usual way of doing
> this is with a physinfo_cap field.
>

I am slightly puzzled by this. I am assuming you are referring to
XENVER_get_features which AFAICT is a stable interface. So why should we
use it to describe the presence of an unstable interface?


> This flag needs using in Patch 10 to reject attempts to create a VM with
> devices when passthrough support is unavailable.
>

May I ask why we can't rely on the hypervisor to reject the flag when
suitable?


> Ian, for the 4.16 release, this series either needs completing with the
> additional flag implemented, or this patch needs reverting to avoid us
> shipping a broken interface.


I fail to see how the interface would be broken... Would you mind to
describe a bit more what could go wrong with this interface?

Cheers,


> ~Andrew
>
>