Re: [PATCH 03/14] KVM: arm64: Continue stage-2 map when re-creating mappings

[Marc Zyngier <maz@kernel.org>]

On Mon, 19 Jul 2021 14:32:10 +0100,
Quentin Perret <qperret@google.com> wrote:
> 
> On Monday 19 Jul 2021 at 13:14:48 (+0100), Marc Zyngier wrote:
> > On Mon, 19 Jul 2021 11:47:24 +0100,
> > Quentin Perret <qperret@google.com> wrote:
> > > 
> > > The stage-2 map walkers currently return -EAGAIN when re-creating
> > > identical mappings or only changing access permissions. This allows to
> > > optimize mapping pages for concurrent (v)CPUs faulting on the same
> > > page.
> > > 
> > > While this works as expected when touching one page-table leaf at a
> > > time, this can lead to difficult situations when mapping larger ranges.
> > > Indeed, a large map operation can fail in the middle if an existing
> > > mapping is found in the range, even if it has compatible attributes,
> > > hence leaving only half of the range mapped.
> > 
> > I'm curious of when this can happen. We normally map a single leaf at
> > a time, and we don't have a way to map multiple leaves at once: we
> > either use the VMA base size or try to upgrade it to a THP, but the
> > result is always a single leaf entry. What changed?
> 
> Nothing _yet_ :-)
> 
> The 'share' hypercall introduced near the end of the series allows to
> share multiple physically contiguous pages in one go -- this is mostly
> to allow sharing data-structures that are larger than a page.
> 
> So if one of the pages happens to be already mapped by the time the
> hypercall is issued, mapping the range with the right SW bits becomes
> difficult as kvm_pgtable_stage2_map() will fail halfway through, which
> is tricky to handle.
> 
> This patch shouldn't change anything for existing users that only map
> things that are nicely aligned at block/page granularity, but should
> make the life of new users easier, so that seemed like a win.

Right, but this is on a different path, right? Guests can never fault
multiple mappings at once, and it takes you a host hypercall to
perform this "multiple leaves at once".

Is there any way we can restrict this to the hypercall? Or even
better, keep the hypercall as a "one page at a time" thing? I can't
imagine it being performance critical (it is a once-off, and only used
over a rather small region of memory). Then, the called doesn't have
to worry about the page already being mapped or not. This would also
match the behaviour of what I do on the MMIO side.

Or do you anticipate much gain from this being able to use block
mappings?

> 
> > > To avoid having to deal with such failures in the caller, don't
> > > interrupt the map operation when hitting existing PTEs, but make sure to
> > > still return -EAGAIN so that user_mem_abort() can mark the page dirty
> > > when needed.
> > 
> > I don't follow you here: if you return -EAGAIN for a writable mapping,
> > we don't account for the page to be dirty on the assumption that
> > nothing has been mapped. But if there is a way to map more than a
> > single entry and to get -EAGAIN at the same time, then we're bound to
> > lose data on page eviction.
> > 
> > Can you shed some light on this?
> 
> Sure. For guests, hitting the -EAGAIN case means we've lost the race
> with another vCPU that faulted the same page. In this case the other
> vCPU either mapped the page RO, which means that our vCPU will then get
> a permission fault next time we run it which will lead to the page being
> marked dirty, or the other vCPU mapped the page RW in which case it
> already marked the page dirty for us and we can safely re-enter the
> guest without doing anything else.
> 
> So what I meant by "still return -EAGAIN so that user_mem_abort() can
> mark the page dirty when needed" is "make sure to mark the page dirty
> only when necessary: if winning the race and marking the page RW, or
> in the permission fault path". That is, by keeping the -EAGAIN I want to
> make sure we don't mark the page dirty twice. (This might fine, but this
> would be new behaviour, and it was not clear that would scale well to
> many vCPUs faulting the same page).
> 
> I see how this wording can be highly confusing though, I'll and re-word
> for the next version.

I indeed found it pretty confusing. A reword would be much appreciated.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.
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