Re: [PATCH] arm64: Restore trapless ptimer access

[Andrew Jones <ajones@ventanamicro.com>]

On Thu, Sep 07, 2023 at 10:31:20PM +0300, Michael Tokarev wrote:
> 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@ ? :)
>

I agree that qemu-trivial should not have been CC'ed for this patch.

Thanks,
drew