linux-kernel - Re: [PATCH v5 2/6] KVM: arm64: Move D-cache flush to the fault handlers

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Message-ID: <YLdiTnfuhMimU4dE@google.com>
Date:   Wed, 2 Jun 2021 10:49:50 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Yanan Wang <wangyanan55@...wei.com>, Will Deacon <will@...nel.org>,
        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 v5 2/6] KVM: arm64: Move D-cache flush to the fault
 handlers

On Wednesday 02 Jun 2021 at 11:19:49 (+0100), Marc Zyngier wrote:
> On Thu, 15 Apr 2021 12:50:28 +0100,
> > @@ -583,6 +589,7 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> >  {
> >  	kvm_pte_t new, old = *ptep;
> >  	u64 granule = kvm_granule_size(level), phys = data->phys;
> > +	struct kvm_pgtable *pgt = data->mmu->pgt;
> >  	struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops;
> >  
> >  	if (!kvm_block_mapping_supported(addr, end, phys, level))
> > @@ -606,6 +613,13 @@ 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 (pgt->flags & KVM_PGTABLE_S2_GUEST) {
> > +		if (stage2_pte_cacheable(pgt, new) && !stage2_has_fwb(pgt))
> > +			__flush_dcache_area(mm_ops->phys_to_virt(phys),
> > +					    granule);
> > +	}
> 
> Rather than this, why not provide new callbacks in mm_ops, even if we
> have to provide one that is specific to guests (and let the protected
> stuff do its own thing)?

Ack, an optional callback in the mm_ops sounds much nicer.

> One thing I really dislike though is that the page-table code is
> starting to be littered with things that are not directly related to
> page tables. We are re-creating the user_mem_abort() mess in a
> different place.

+1, we should probably keep the page-table code as close as possible
to a standalone and architecturally-compliant library as opposed to a
mess of unrelated logic, simply because that will lead to a cleaner and
more maintainable design in the long run, and because that will ease the
interoperability with EL2 in protected mode.

Thanks,
Quentin