linux-kernel - Re: [PATCH v7 4/4] KVM: arm64: Move guest CMOs to the fault handlers

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Date:   Thu, 17 Jun 2021 14:21:16 +0100
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Yanan Wang <wangyanan55@...wei.com>,
        Quentin Perret <qperret@...gle.com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Gavin Shan <gshan@...hat.com>, wanghaibin.wang@...wei.com,
        zhukeqian1@...wei.com, yuzenghui@...wei.com
Subject: Re: [PATCH v7 4/4] KVM: arm64: Move guest CMOs to the fault handlers

On Thu, Jun 17, 2021 at 01:59:37PM +0100, Marc Zyngier wrote:
> On Thu, 17 Jun 2021 13:45:57 +0100,
> Will Deacon <will@...nel.org> wrote:
> > On Thu, Jun 17, 2021 at 06:58:24PM +0800, Yanan Wang wrote:
> > > @@ -606,6 +618,14 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> > >  		stage2_put_pte(ptep, data->mmu, addr, level, mm_ops);
> > >  	}
> > >  
> > > +	/* Perform CMOs before installation of the guest stage-2 PTE */
> > > +	if (mm_ops->clean_invalidate_dcache && stage2_pte_cacheable(pgt, new))
> > > +		mm_ops->clean_invalidate_dcache(kvm_pte_follow(new, mm_ops),
> > > +						granule);
> > > +
> > > +	if (mm_ops->invalidate_icache && stage2_pte_executable(new))
> > > +		mm_ops->invalidate_icache(kvm_pte_follow(new, mm_ops), granule);
> > 
> > One thing I'm missing here is why we need the indirection via mm_ops. Are
> > there cases where we would want to pass a different function pointer for
> > invalidating the icache? If not, why not just call the function directly?
> > 
> > Same for the D side.
> 
> If we didn't do that, we'd end-up having to track whether the guest
> context requires CMOs with additional flags, which is pretty ugly (see
> v5 of this series for reference [1]).

Fair enough, although the function pointers here _are_ being used as flags,
as they only ever have one of two possible values (NULL or the CMO function),
so it's a shame to bring in the indirect branch as well.

> It also means that we would have to drag the CM functions into the EL2
> object, something that we don't need with this approach.

I think it won't be long before we end up with CMO functions at EL2 and
you'd hope we'd be able to use the same code as EL1 for something like
that. But I also wouldn't want to put money on it...

Anyway, no strong opinion on this, it just jumped out when I skimmed the
patches.

Will