Re: [XEN PATCH v11 2/8] x86/pvh: Allow (un)map_pirq when dom0 is PVH

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.06.2024 14:33, 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 removal device path to unmap pirq.
> And add a new check to prevent (un)map when the subject domain
> has no X86_EMU_USE_PIRQ flag.
> 
> So that the interrupt of a passthrough device can be
> successfully mapped to pirq for domU with X86_EMU_USE_PIRQ flag
> when dom0 is PVH
> 
> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

You keep carrying this R-b, despite making functional changes. This can't be
quite right.

While functionally I'm now okay with the change, I still have a code structure
concern:

> --- a/xen/arch/x86/physdev.c
> +++ b/xen/arch/x86/physdev.c
> @@ -323,6 +323,13 @@ ret_t do_physdev_op(int cmd, 
> XEN_GUEST_HANDLE_PARAM(void) arg)
>          if ( !d )
>              break;
>  
> +        /* Prevent mapping when the subject domain has no X86_EMU_USE_PIRQ */
> +        if ( is_hvm_domain(d) && !has_pirq(d) )
> +        {
> +            rcu_unlock_domain(d);
> +            return -EOPNOTSUPP;
> +        }
> +
>          ret = physdev_map_pirq(d, map.type, &map.index, &map.pirq, &msi);
>  
>          rcu_unlock_domain(d);
> @@ -346,6 +353,13 @@ ret_t do_physdev_op(int cmd, 
> XEN_GUEST_HANDLE_PARAM(void) arg)
>          if ( !d )
>              break;
>  
> +        /* Prevent unmapping when the subject domain has no X86_EMU_USE_PIRQ 
> */
> +        if ( is_hvm_domain(d) && !has_pirq(d) )
> +        {
> +            rcu_unlock_domain(d);
> +            return -EOPNOTSUPP;
> +        }
> +
>          ret = physdev_unmap_pirq(d, unmap.pirq);
>  
>          rcu_unlock_domain(d);

If you did go look, you will have noticed that we use "return" in the middle
of this function only very sparingly (when alternatives would result in more
complicated code elsewhere). I think you want to avoid "return" here, too,
and probably go even further and avoid the extra rcu_unlock_domain() as well.
That's easily possible to arrange for (taking the latter case as example):

        /* Prevent unmapping when the subject domain has no X86_EMU_USE_PIRQ */
        if ( !is_hvm_domain(d) || has_pirq(d) )
            ret = physdev_unmap_pirq(d, unmap.pirq);
        else
            ret = -EOPNOTSUPP;

        rcu_unlock_domain(d);

Personally I would even use a conditional operator here, but I believe
others might dislike its use in situations like this one.

The re-arrangement make a little more noticeable though that the comment
isn't quite right either: PV domains necessarily have no
X86_EMU_USE_PIRQ. Maybe "... has no notion of pIRQ"?

Jan