Re: [PATCH] arm64: Restore trapless ptimer access

[Andrew Jones <andrew.jones@linux.dev>]

On Thu, Aug 31, 2023 at 07:00:52PM +0000, 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

The link can be shorter with

https://lore.kernel.org/all/20230823200408.1214332-1-coltonlewis@google.com/

> for additional context.
> 
> Signed-off-by: Andrew Jones <andrew.jones@linux.dev>

Thanks for the testing and posting, Colton. Please add your s-o-b and a
Tested-by tag as well.

Thanks,
drew

> ---
>  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)
> -- 
> 2.42.0.283.g2d96d420d3-goog
>