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

From: Peter Zijlstra <peterz@infradead.org>
To: Jon Kohler <jon@nutanix.com>
Cc: Babu Moger <babu.moger@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Yu-cheng Yu <yu-cheng.yu@intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Tony Luck <tony.luck@intel.com>, Uros Bizjak <ubizjak@gmail.com>,
	Petteri Aimonen <jpa@git.mail.kapsi.fi>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Kan Liang <kan.liang@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Rapoport <rppt@kernel.org>, Fan Yang <Fan_Yang@sjtu.edu.cn>,
	Juergen Gross <jgross@suse.com>,
	Benjamin Thiel <b.thiel@posteo.de>,
	Dave Jiang <dave.jiang@intel.com>,
	Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v3] KVM: x86: use wrpkru directly in kvm_load_{guest|host}_xsave_state
Date: Wed, 12 May 2021 09:41:14 +0200	[thread overview]
Message-ID: <YJuGms6UnRVpP7U/@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210511170508.40034-1-jon@nutanix.com>

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.