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

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Message-ID: <7fedbeac-300f-48a3-9860-e05b6d286cd1@xen.org>
Date: Mon, 12 May 2025 10:29:26 +0100
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.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 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>