Re: [PATCH] arm64: Restore trapless ptimer access

[Colton Lewis <coltonlewis@google.com>]

On Fri, Sep 01, 2023 at 09:35:47AM +0200, Andrew Jones wrote:
> 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/

I will keep that in mind next time.

> > 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.

Assuming it is sufficient to add here instead of reposting the whole patch:

Signed-off-by: Colton Lewis <coltonlewis@google.com>
Tested-by: Colton Lewis <coltonlewis@google.com>

> > ---
> >  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
> >