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

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Message-ID: <YXBmoP4Lf2o1OiHY@google.com>
Date:   Wed, 20 Oct 2021 18:57:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
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 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 :-)