Re: KVM: x86/xen: Allow 'out of range' event channel ports in IRQ routing table.

[Paul Durrant <xadimgnik@gmail.com>]

On 08/05/2025 21:30, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> To avoid imposing an ordering constraint on userspace, allow 'invalid'
> event channel targets to be configured in the IRQ routing table.
> 
> This is the same as accepting interrupts targeted at vCPUs which don't
> exist yet, which is already the case for both Xen event channels *and*
> for MSIs (which don't do any filtering of permitted APIC ID targets at
> all).
> 
> If userspace actually *triggers* an IRQ with an invalid target, that
> will fail cleanly, as kvm_xen_set_evtchn_fast() also does the same range
> check.
> 
> If KVM enforced that the IRQ target must be valid at the time it is
> *configured*, that would force userspace to create all vCPUs and do
> various other parts of setup (in this case, setting the Xen long_mode)
> before restoring the IRQ table.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   arch/x86/kvm/xen.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
> 

Reviewed-by: Paul Durrant <paul@xen.org>