linux-kernel - Re: [PATCH v5 07/13] KVM: Just resync arch fields when slots_arch

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Message-ID: <7e68e883-7431-72a3-82ef-306472de5ac4@maciej.szmigiero.name>
Date:   Wed, 20 Oct 2021 20:58:41 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 07/13] KVM: Just resync arch fields when
 slots_arch_lock gets reacquired

On 20.10.2021 20:57, Sean Christopherson wrote:
> On Wed, Oct 20, 2021, Maciej S. Szmigiero wrote:
>> On 20.10.2021 01:55, Sean Christopherson wrote:
>>> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
>>> This should probably be a memcpy(), I don't know what all shenanigans the compiler
>>> can throw at us if it gets to copy a struct by value.
>>
>> Normally, copy-assignment of a struct is a safe operation (this is purely
>> an internal kernel struct, so there are no worries about padding leakage
>> to the userspace), but can replace this with a memcpy().
> 
> I was more worried about the compiler using SIMD instructions.  I assume the kernel
> build process has lots of guards in place to prevent such shenanigans, but on the
> other hand I _know_ mempcy() is safe :-)
> 

So we will play safe and use mempcy() then :)

Thanks,
Maciej