Re: [PATCH] hw/intc/arm_gicv3_kvm: Stop wrongly programming GICR_PENDBASER.PTZ bit

[Peter Maydell <peter.maydell@linaro.org>]

On Sun, 19 Jan 2020 at 13:32, Zenghui Yu <yuzenghui@huawei.com> wrote:
>
> If LPIs are disabled, KVM will just ignore the GICR_PENDBASER.PTZ bit when
> restoring GICR_CTLR.  Setting PTZ here makes littlt sense in "reduce GIC
> initialization time".
>
> And what's worse, PTZ is generally programmed by guest to indicate to the
> Redistributor whether the LPI Pending table is zero when enabling LPIs.
> If migration is triggered when the PTZ has just been cleared by guest (and
> before enabling LPIs), we will see PTZ==1 on the destination side, which
> is not as expected.  Let's just drop this hackish userspace behavior.
>
> Also take this chance to refine the comment a bit.

I pulled up the original code-review discussion where this code
was added:
https://lists.gnu.org/archive/html/qemu-devel/2015-10/msg05889.html

It looks like the original reason for adding it was that the
GICR_PENDBASER.PTZ bit is write-only, and so there's no way for
the migration code in QEMU to accurately restore it on the
destination end of a migration. So Pavel picked a heuristic
for guessing what the right value of the PTZ bit was.

It sounds like the penalty if userspace sets the bit to 0
always on a migration is not significant, so I guess that
just setting this bit to 0 always is an OK thing to do (as you
say, there's a window in the existing implementation where
we do the wrong thing sometimes if migration happens at the
wrong time, because the current code's heuristic can
sometimes fail in the wrong direction).

Applied to target-arm.next.

thanks
-- PMM