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Date:   Wed, 12 May 2021 09:41:14 +0200
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
Message-ID: <YJuGms6UnRVpP7U/@hirez.programming.kicks-ass.net>
References: <20210511170508.40034-1-jon@nutanix.com>
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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.