Re: [PATCH] arm64: Restore trapless ptimer access

[Michael Tokarev]

31.08.2023 22:00, Colton Lewis 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>
> ---
>   target/arm/kvm64.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/target/arm/kvm64.c b/target/arm/kvm64.c
> index 4d904a1d11..2dd46e0a99 100644
> --- a/target/arm/kvm64.c
> +++ b/target/arm/kvm64.c
> @@ -672,6 +672,7 @@ typedef struct CPRegStateLevel {
>    */
>   static const CPRegStateLevel non_runtime_cpregs[] = {
>       { KVM_REG_ARM_TIMER_CNT, KVM_PUT_FULL_STATE },
> +    { KVM_REG_ARM_PTIMER_CNT, KVM_PUT_FULL_STATE },
>   };
>   
>   int kvm_arm_cpreg_level(uint64_t regidx)

While this patch itself is one-liner and trivial and all, I'd rather
not apply this to the trivial-patches tree, - it requires a little
bit more than trivial expertise in this area.

So basically, ping for qemu-arm@ ? :)

Thanks,

/mjt