Re: [PATCH] arm64: Restore trapless ptimer access

[Peter Maydell]

On Thu, 31 Aug 2023 at 20:01, Colton Lewis <coltonlewis@google.com> wrote:
>
> Due to recent KVM changes, QEMU is setting a ptimer offset resulting
> in unintended trap and emulate access and a consequent performance
> hit. Filter out the PTIMER_CNT register to restore trapless ptimer
> access.
>
> Quoting Andrew Jones:
>
> Simply reading the CNT register and writing back the same value is
> enough to set an offset, since the timer will have certainly moved
> past whatever value was read by the time it's written.  QEMU
> frequently saves and restores all registers in the get-reg-list array,
> unless they've been explicitly filtered out (with Linux commit
> 680232a94c12, KVM_REG_ARM_PTIMER_CNT is now in the array). So, to
> restore trapless ptimer accesses, we need a QEMU patch to filter out
> the register.
>
> See
> https://lore.kernel.org/kvmarm/gsntttsonus5.fsf@coltonlewis-kvm.c.googlers.com/T/#m0770023762a821db2a3f0dd0a7dc6aa54e0d0da9
> for additional context.
>
> Signed-off-by: Andrew Jones <andrew.jones@linux.dev>
> ---



Applied to target-arm.next, thanks.

-- PMM