Re: [PATCH v3] KVM: x86: use wrpkru directly in kvm_load_{guest|host}_xsave_state

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, May 11, 2021 at 01:05:02PM -0400, Jon Kohler wrote:
> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> index 8d33ad80704f..5bc4df3a4c27 100644
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -583,7 +583,13 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
>  		if (pk)
>  			pkru_val = pk->pkru;
>  	}
> -	__write_pkru(pkru_val);
> +
> +	/*
> +	 * WRPKRU is relatively expensive compared to RDPKRU.
> +	 * Avoid WRPKRU when it would not change the value.
> +	 */
> +	if (pkru_val != rdpkru())
> +		wrpkru(pkru_val);

Just wondering; why aren't we having that in a per-cpu variable? The
usual per-cpu MSR shadow approach avoids issuing any 'special' ops
entirely.