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

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Message-ID: <aFmFYZlrxlLE2ZzY@google.com>
Date: Mon, 23 Jun 2025 09:48:33 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: paul@....org, kvm@...r.kernel.org, Ivan Orlov <iorlov@...zon.co.uk>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: KVM: x86/xen: Allow 'out of range' event channel ports in IRQ
 routing table.

On Mon, Jun 23, 2025, David Woodhouse wrote:
> On Mon, 2025-05-12 at 10:29 +0100, Paul Durrant wrote:
> > On 08/05/2025 21:30, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@...zon.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@...r.kernel.org
> > > Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> > > ---
> > >   arch/x86/kvm/xen.c | 14 ++++++++++++--
> > >   1 file changed, 12 insertions(+), 2 deletions(-)
> > > 
> > 
> > Reviewed-by: Paul Durrant <paul@....org>
> 
> Ping?

Almost there :-)  I've got it applied (for 6.16), just need to run my generic
test stuff before making it "official".