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Date:   Thu, 22 Jul 2021 10:11:58 +0100
From:   Quentin Perret <qperret@google.com>
To:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
Cc:     Will Deacon <will@kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "maz@kernel.org" <maz@kernel.org>,
        "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
        "james.morse@arm.com" <james.morse@arm.com>,
        "julien.thierry.kdev@gmail.com" <julien.thierry.kdev@gmail.com>,
        "suzuki.poulose@arm.com" <suzuki.poulose@arm.com>,
        "jean-philippe@linaro.org" <jean-philippe@linaro.org>,
        "Alexandru.Elisei@arm.com" <Alexandru.Elisei@arm.com>,
        Linuxarm <linuxarm@huawei.com>
Subject: Re: [PATCH v2 3/3] kvm/arm: Align the VMID allocation with the arm64
 ASID one
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On Thursday 22 Jul 2021 at 06:45:14 (+0000), Shameerali Kolothum Thodi wrote:
> > From: Will Deacon [mailto:will@kernel.org]
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > > index 4b60c0056c04..a02c4877a055 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > > @@ -106,8 +106,7 @@ int kvm_host_prepare_stage2(void *mem_pgt_pool,
> > void *dev_pgt_pool)
> > >  	mmu->pgd_phys = __hyp_pa(host_kvm.pgt.pgd);
> > >  	mmu->arch = &host_kvm.arch;
> > >  	mmu->pgt = &host_kvm.pgt;
> > > -	mmu->vmid.vmid_gen = 0;
> > > -	mmu->vmid.vmid = 0;
> > > +	atomic64_set(&mmu->vmid.id, 0);
> > 
> > I think this is the first atomic64 use in the EL2 object, which may pull in
> > some fatal KCSAN instrumentation. Quentin, have you run into this before?
> > 
> > Might be simple just to zero-initialise mmu for now, if it isn't already.
> 
> I will check that.

Yes I think what saves us here is that, AFAICT. arm64 doesn't support
KCSAN yet. But the day it does, this should fail to link (hopefully)
because of out-of-line calls into e.g. __kasan_check_write().

So yes, a simple zeroing here is probably preferable.

Thanks,
Quentin