Re: [PATCH v2 1/3] KVM: PPC: Book3S HV: Fix race in re-enabling XIVE escalation interrupts

[Michael Ellerman <patch-notifications@ellerman.id.au>]

On Tue, 2019-08-13 at 10:03:49 UTC, Paul Mackerras wrote:
> Escalation interrupts are interrupts sent to the host by the XIVE
> hardware when it has an interrupt to deliver to a guest VCPU but that
> VCPU is not running anywhere in the system.  Hence we disable the
> escalation interrupt for the VCPU being run when we enter the guest
> and re-enable it when the guest does an H_CEDE hypercall indicating
> it is idle.
> 
> It is possible that an escalation interrupt gets generated just as we
> are entering the guest.  In that case the escalation interrupt may be
> using a queue entry in one of the interrupt queues, and that queue
> entry may not have been processed when the guest exits with an H_CEDE.
> The existing entry code detects this situation and does not clear the
> vcpu->arch.xive_esc_on flag as an indication that there is a pending
> queue entry (if the queue entry gets processed, xive_esc_irq() will
> clear the flag).  There is a comment in the code saying that if the
> flag is still set on H_CEDE, we have to abort the cede rather than
> re-enabling the escalation interrupt, lest we end up with two
> occurrences of the escalation interrupt in the interrupt queue.
> 
> However, the exit code doesn't do that; it aborts the cede in the sense
> that vcpu->arch.ceded gets cleared, but it still enables the escalation
> interrupt by setting the source's PQ bits to 00.  Instead we need to
> set the PQ bits to 10, indicating that an interrupt has been triggered.
> We also need to avoid setting vcpu->arch.xive_esc_on in this case
> (i.e. vcpu->arch.xive_esc_on seen to be set on H_CEDE) because
> xive_esc_irq() will run at some point and clear it, and if we race with
> that we may end up with an incorrect result (i.e. xive_esc_on set when
> the escalation interrupt has just been handled).
> 
> It is extremely unlikely that having two queue entries would cause
> observable problems; theoretically it could cause queue overflow, but
> the CPU would have to have thousands of interrupts targetted to it for
> that to be possible.  However, this fix will also make it possible to
> determine accurately whether there is an unhandled escalation
> interrupt in the queue, which will be needed by the following patch.
> 
> Cc: stable@vger.kernel.org # v4.16+
> Fixes: 9b9b13a6d153 ("KVM: PPC: Book3S HV: Keep XIVE escalation interrupt masked unless ceded")
> Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

Applied to powerpc topic/ppc-kvm, thanks.

https://git.kernel.org/powerpc/c/959c5d5134786b4988b6fdd08e444aa67d1667ed

cheers