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

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Sep 26, 2023 at 04:58:03PM +0100, Catalin Marinas wrote:
> On Fri, Sep 22, 2023 at 04:59:08PM +0000, Oliver Upton wrote:
> > 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).
> 
> When I did the stage 2 AF support I left out DBM as it was unlikely
> to be of any use in the real world. Now with dirty tracking for
> migration, we may have a better use for this feature.
> 
> What I find confusing with these patches is that stage2_wp_range() is
> supposed to make a stage 2 pte read-only, as the name implies. However,
> if the pte was writeable, it leaves it writeable, clean with DBM
> enabled. Doesn't the change to kvm_pgtable_stage2_wrprotect() in patch 4
> break other uses of stage2_wp_range()? E.g. kvm_mmu_wp_memory_region()?

Ah, that's also used for dirty tracking, so maybe it's ok.

AFAICT KVM doesn't do any form of stage 2 pte change from writeable to
read-only other than dirty tracking (all other cases triggered via MMU
notifier end up unmapping at stage 2).

> Unless I misunderstood, I'd rather change
> kvm_arch_mmu_enable_log_dirty_pt_masked() to call a new function,
> stage2_clean_range(), which clears S2AP[1] together with setting DBM if
> previously writeable. But we should not confuse this with
> write-protecting or change the write-protecting functions to mark a pte
> writeable+clean.

I think it's still good to rename stage2_wp_range() to make it clear
that it's about clean ptes rather than read-only.

-- 
Catalin