Re: [XEN PATCH v9 2/5] x86/pvh: Allow (un)map_pirq when dom0 is PVH

[Jan Beulich <jbeulich@suse.com>]

On 07.06.2024 10:11, Jiqian Chen wrote:
> If run Xen with PVH dom0 and hvm domU, hvm will map a pirq for
> a passthrough device by using gsi, see qemu code
> xen_pt_realize->xc_physdev_map_pirq and libxl code
> pci_add_dm_done->xc_physdev_map_pirq. Then xc_physdev_map_pirq
> will call into Xen, but in hvm_physdev_op, PHYSDEVOP_map_pirq
> is not allowed because currd is PVH dom0 and PVH has no
> X86_EMU_USE_PIRQ flag, it will fail at has_pirq check.
> 
> So, allow PHYSDEVOP_map_pirq when dom0 is PVH and also allow
> PHYSDEVOP_unmap_pirq for the failed path to unmap pirq. And
> add a new check to prevent self map when subject domain has no
> PIRQ flag.
> 
> Signed-off-by: Huang Rui <ray.huang@amd.com>
> Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

What's imo missing in the description is a clarification / justification of
why it is going to be a good idea (or at least an acceptable one) to expose
the concept of PIRQs to PVH. If I'm not mistaken that concept so far has
been entirely a PV one.

> --- a/xen/arch/x86/hvm/hypercall.c
> +++ b/xen/arch/x86/hvm/hypercall.c
> @@ -71,8 +71,14 @@ long hvm_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>  
>      switch ( cmd )
>      {
> +    /*
> +     * Only being permitted for management of other domains.
> +     * Further restrictions are enforced in do_physdev_op.
> +     */
>      case PHYSDEVOP_map_pirq:
>      case PHYSDEVOP_unmap_pirq:
> +        break;

Nit: Imo such a comment ought to be indented like code (statements), not
like the case labels.

Jan