Re: [PATCH v5 07/11] xen/domctl: Introduce XEN_DOMCTL_CDF_vpci flag

[Julien Grall <julien.grall.oss@xxxxxxxxx>]

Hi Andrew,

On Fri, 8 Oct 2021, 20:07 Andrew Cooper, <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxx>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Acked-by: Christian Lindig <christian.lindig@xxxxxxxxxx>

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