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Subject: Re: [PATCH v3] KVM: x86: use wrpkru directly in
 kvm_load_{guest|host}_xsave_state
To:     Peter Zijlstra <peterz@infradead.org>, 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>,
        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
References: <20210511170508.40034-1-jon@nutanix.com>
 <YJuGms6UnRVpP7U/@hirez.programming.kicks-ass.net>
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <25d39a79-d8d8-9798-a930-ccdace304bac@intel.com>
Date:   Wed, 12 May 2021 11:33:21 -0700
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Precedence: bulk

On 5/12/21 12:41 AM, Peter Zijlstra wrote:
> 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.

It could be a per-cpu variable.  When I wrote this originally I figured
that a rdpkru would be cheaper than a load from memory (even per-cpu
memory).

But, now that I think about it, assuming that 'prku_val' is in %rdi, doing:

	cmp	%gs:0x1234, %rdi

might end up being cheaper than clobbering a *pair* of GPRs with rdpkru:

	xor    %ecx,%ecx
	rdpkru
	cmp	%rax, %rdi

I'm too lazy to go figure out what would be faster in practice, though.
 Does anyone care?