Subject: Re: [PATCH v2] KVM: PPC: Book3S HV nestedv2: Cancel pending DEC exception

From: Michael Ellerman <[email-masked]>
Date: Mon, 15 Apr 2024 22:27:32 +1000

Vaibhav Jain <[email-masked]> writes:
> This reverts commit 180c6b072bf3 ("KVM: PPC: Book3S HV nestedv2: Do not
> cancel pending decrementer exception") [1] which prevented canceling a
> pending HDEC exception for nestedv2 KVM guests. It was done to avoid
> overhead of a H_GUEST_GET_STATE hcall to read the 'DEC expiry TB' register
> which was higher compared to handling extra decrementer exceptions.
>
> However recent benchmarks indicate that overhead of not handling 'DECR'
> expiry for Nested KVM Guest(L2) is higher and results in much larger exits
> to Pseries Host(L1) as indicated by the Unixbench-arithoh bench[2]

Any reason you chose that benchmark? At least on my system it seems to
compile to an infinite loop incrementing a single register.

Presumably the change is still good, but a more well known benchmark
would be good, even if it's just stress-ng, at least that's a bit more
standard.

cheers

> Metric	    	      | Current upstream    | Revert [1]  | Difference %
> ========================================================================
> arithoh-count (10)    |	3244831634	    | 3403089673  | +04.88%
> kvm_hv:kvm_guest_exit |	513558		    | 152441	  | -70.32%
> probe:kvmppc_gsb_recv |	28060		    | 28110	  | +00.18%
>
> N=1
>
> As indicated by the data above that reverting [1] results in substantial
> reduction in number of L2->L1 exits with only slight increase in number of
> H_GUEST_GET_STATE hcalls to read the value of 'DEC expiry TB'. This results
> in an overall ~4% improvement of arithoh[2] throughput.
>
> [1] commit 180c6b072bf3 ("KVM: PPC: Book3S HV nestedv2: Do not cancel pending decrementer exception")
> [2] https://github.com/kdlucas/byte-unixbench/
>
> Fixes: 180c6b072bf3 ("KVM: PPC: Book3S HV nestedv2: Do not cancel pending decrementer exception")
> Signed-off-by: Vaibhav Jain <[email-masked]>
>
> ---
> Changelog:
> Since v1: https://lore.kernel.org/all/[email-masked]
> * Updated/Corrected patch title and description
> * Included data on test benchmark results for Unixbench-arithoh bench.
> ---
>  arch/powerpc/kvm/book3s_hv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 8e86eb577eb8..692a7c6f5fd9 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -4857,7 +4857,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
>  	 * entering a nested guest in which case the decrementer is now owned
>  	 * by L2 and the L1 decrementer is provided in hdec_expires
>  	 */
> -	if (!kvmhv_is_nestedv2() && kvmppc_core_pending_dec(vcpu) &&
> +	if (kvmppc_core_pending_dec(vcpu) &&
>  			((tb < kvmppc_dec_expires_host_tb(vcpu)) ||
>  			 (trap == BOOK3S_INTERRUPT_SYSCALL &&
>  			  kvmppc_get_gpr(vcpu, 3) == H_ENTER_NESTED)))
> -- 
> 2.44.0