[PATCH v2 0/2] KVM: arm64: vgic-its: Fix LPI double mapping and DISCARD

[Fuad Tabba]

Hi Marc,

On Tue, 13 Oct 2026 11:01:44 +0100, Marc Zyngier <maz at kernel.org> wrote:
[...]
> Yeah, I now have a series that has the counting pause, your single
> vcpu pause (fixed to DTRT), and the LPI disabling fix. Patch #2 of
> this series can probably make use of the single-vcpu stunning.

I'll rebase the series on that once it's out, with patch 2 using the
single-vcpu stunning.

> As for the OUTSIDE_GUEST discussion, I've had a long discussion with
> Will where we reversed engineered the synchronisation requirements,
> and concluded that there was nothing to fix on that front (the logic
> is incredibly subtle and counter-intuitive, but overall sound). This
> patch is therefore dropped.

The first [High] issue Sashiko found on patch 2 [1] holds for DISCARD,
because the fold writes back the pending_latch that DISCARD clears.
With a 2ms delay after the exit to widen the window, and the DISCARD
sent at a varying time, the guest in my DISCARD reproducer takes the
discarded LPI in 13 of 20 runs on v2.

A vCPU on its way into the guest does the same: it loads its LRs in
kvm_vgic_flush_hwstate() while it's still OUTSIDE_GUEST_MODE, so
kvm_arm_halt_guest() doesn't wait for it. With the delay after the
flush instead, on v2 plus your 1/7 and 2/7 [2] and both changes from
your reply to 1/7 [3], the guest takes it in 9 of 20 runs.

Once DISCARD waits for irq->on_lr to clear before clearing
pending_latch, the guest takes it in none of 20 runs in either case.
I'll add that to v3, unless the single-vcpu stunning already waits for
it.

Cheers,
/fuad

[1] https://lore.kernel.org/r/sashiko-outbox-169278@kernel.org
[2] https://lore.kernel.org/r/20260929093548.3598547-1-maz@kernel.org
[3] https://lore.kernel.org/r/86a4p0403b.wl-maz@kernel.org