linux-kernel - Re: [PATCH v2] KVM: x86: use wrpkru directly in kvm_load_{guest|host}_xsave

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Message-ID: <95947b5c-9add-e5d3-16a5-a40ab6d24978@redhat.com>
Date:   Tue, 11 May 2021 19:08:06 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>, Jon Kohler <jon@...anix.com>
Cc:     Babu Moger <babu.moger@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Petteri Aimonen <jpa@....mail.kapsi.fi>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Uros Bizjak <ubizjak@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Benjamin Thiel <b.thiel@...teo.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Juergen Gross <jgross@...e.com>,
        Fan Yang <Fan_Yang@...u.edu.cn>,
        Dave Jiang <dave.jiang@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: use wrpkru directly in
 kvm_load_{guest|host}_xsave_state

On 11/05/21 18:45, Dave Hansen wrote:
> On 5/11/21 8:59 AM, Jon Kohler wrote:
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index b1099f2d9800..20f1fb8be7ef 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -151,7 +151,7 @@ static inline void write_pkru(u32 pkru)
>>   	fpregs_lock();
>>   	if (pk)
>>   		pk->pkru = pkru;
>> -	__write_pkru(pkru);
>> +	wrpkru(pkru);
>>   	fpregs_unlock();
>>   }
> 
> This removes the:
> 
> 	if (pkru == rdpkru())
> 		return;
> 
> optimization from a couple of write_pkru() users:
> arch_set_user_pkey_access() and copy_init_pkru_to_fpregs().
> 
> Was that intentional?  Those aren't the hottest paths in the kernel, but
> copy_init_pkru_to_fpregs() is used in signal handling and exeve().

Yeah, you should move it from __write_pkru() to write_pkru() but not 
remove it completely.

Paolo