Re: [RFC PATCH v2 6/8] KVM: arm64: Only write protect selected PTE

[Oliver Upton <oliver.upton@linux.dev>]

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