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From:   Oliver Upton <oliver.upton@linux.dev>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>,
        kvmarm@lists.linux.dev, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, maz@kernel.org,
        will@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com,
        yuzenghui@huawei.com, zhukeqian1@huawei.com,
        jonathan.cameron@huawei.com, linuxarm@huawei.com
Subject: Re: [RFC PATCH v2 6/8] KVM: arm64: Only write protect selected PTE
Message-ID: <ZQ3H3JZHnxIVCIF6@linux.dev>
References: <20230825093528.1637-1-shameerali.kolothum.thodi@huawei.com>
 <20230825093528.1637-7-shameerali.kolothum.thodi@huawei.com>
 <ZQ26KE2bzEFYUpMc@arm.com>
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On Fri, Sep 22, 2023 at 05:00:40PM +0100, Catalin Marinas wrote:
> On Fri, Aug 25, 2023 at 10:35:26AM +0100, Shameer Kolothum wrote:
> > From: Keqian Zhu <zhukeqian1@huawei.com>
> > 
> > This function write protects all PTEs between the ffs and fls of mask.
> > There may be unset bits between this range. It works well under pure
> > software dirty log, as software dirty log is not working during this
> > process.
> > 
> > But it will unexpectly clear dirty status of PTE when hardware dirty
> > log is enabled. So change it to only write protect selected PTE.
> 
> Ah, I did wonder about losing the dirty status. The equivalent to S1
> would be for kvm_pgtable_stage2_wrprotect() to set a software dirty bit.
> 
> I'm only superficially familiar with how KVM does dirty tracking for
> live migration. Does it need to first write-protect the pages and
> disable DBM? Is DBM re-enabled later? Or does stage2_wp_range() with
> your patches leave the DBM on? If the latter, the 'wp' aspect is a bit
> confusing since DBM basically means writeable (and maybe clean). So
> better to have something like stage2_clean_range().

KVM has never enabled DBM and we solely rely on write-protection faults
for dirty tracking. IOW, we do not have a writable-clean state for
stage-2 PTEs (yet).

-- 
Thanks,
Oliver