linux-kernel - Re: [PATCH v2 5/6] kvm: allocate vcpu pointer array separately

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Date:   Thu, 9 Sep 2021 20:28:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Juergen Gross <jgross@...e.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, ehabkost@...hat.com,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2 5/6] kvm: allocate vcpu pointer array separately

On Mon, Sep 06, 2021, Marc Zyngier wrote:
> On Mon, 06 Sep 2021 05:33:35 +0100,
> Juergen Gross <jgross@...e.com> wrote:
> > 
> > On 03.09.21 16:41, Marc Zyngier wrote:
> >
> > > At this stage, I really wonder why we are not using an xarray instead.
> > > 
> > > I wrote this [1] a while ago, and nothing caught fire. It was also a
> > > net deletion of code...
> > 
> > Indeed, I'd prefer that solution!
> > 
> > Are you fine with me swapping my patch with yours in the series?
> 
> Of course, feel free to grab the whole series. You'll probably need
> the initial patches to set the scene for this. On their own, they are
> a nice cleanup, and I trust you can write a decent commit message for
> the three patches affecting mips/s390/x86.

It would also be a good idea to convert kvm_for_each_vcpu() to use xa_for_each(),
I assume that's more performant than 2x atomic_read() + xa_load().  Unless I'm
misreading the code, xa_for_each() is guaranteed to iterate in ascending index
order, i.e. preserves the current vcpu0..vcpuN iteration order.