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

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Message-ID: <7cc618b80681e8e1402c73886505f6247c810db8.camel@infradead.org>
Date: Mon, 23 Jun 2025 18:44:26 +0200
From: David Woodhouse <dwmw2@...radead.org>
To: paul@....org, kvm@...r.kernel.org, Sean Christopherson
 <seanjc@...gle.com>,  "Orlov, Ivan" <iorlov@...zon.co.uk>
Cc: 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, 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>

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