Re: [PATCH v5 0/8] KVM: arm64: permit MAP_SHARED mappings with MTE enabled

[Marc Zyngier <maz@kernel.org>]

On Fri, 04 Nov 2022 17:42:27 +0000,
Peter Collingbourne <pcc@google.com> wrote:
> 
> On Fri, Nov 4, 2022 at 9:23 AM Marc Zyngier <maz@kernel.org> wrote:
> >
> > On Fri, 04 Nov 2022 01:10:33 +0000,
> > Peter Collingbourne <pcc@google.com> wrote:
> > >
> > > Hi,
> > >
> > > This patch series allows VMMs to use shared mappings in MTE enabled
> > > guests. The first five patches were taken from Catalin's tree [1] which
> > > addressed some review feedback from when they were previously sent out
> > > as v3 of this series. The first patch from Catalin's tree makes room
> > > for an additional PG_arch_3 flag by making the newer PG_arch_* flags
> > > arch-dependent. The next four patches are based on a series that
> > > Catalin sent out prior to v3, whose cover letter [2] I quote from below:
> > >
> > > > This series aims to fix the races between initialising the tags on a
> > > > page and setting the PG_mte_tagged flag. Currently the flag is set
> > > > either before or after that tag initialisation and this can lead to CoW
> > > > copying stale tags. The first patch moves the flag setting after the
> > > > tags have been initialised, solving the CoW issue. However, concurrent
> > > > mprotect() on a shared mapping may (very rarely) lead to valid tags
> > > > being zeroed.
> > > >
> > > > The second skips the sanitise_mte_tags() call in kvm_set_spte_gfn(),
> > > > deferring it to user_mem_abort(). The outcome is that no
> > > > sanitise_mte_tags() can be simplified to skip the pfn_to_online_page()
> > > > check and only rely on VM_MTE_ALLOWED vma flag that can be checked in
> > > > user_mem_abort().
> > > >
> > > > The third and fourth patches use PG_arch_3 as a lock for page tagging,
> > > > based on Peter Collingbourne's idea of a two-bit lock.
> > > >
> > > > I think the first patch can be queued but the rest needs some in depth
> > > > review and test. With this series (if correct) we could allos MAP_SHARED
> > > > on KVM guest memory but this is to be discussed separately as there are
> > > > some KVM ABI implications.
> > >
> > > In this v5 I rebased Catalin's tree onto -next again. Please double check
> >
> > Please don't do use -next as a base. In-flight series should be based
> > on a *stable* tag, either 6.0 or one of the early -RCs. If there is a
> > known conflict with -next, do mention it in the cover letter and
> > provide a resolution.
> 
> Okay, I will keep that in mind.
> 
> > > my rebase, which resolved the conflict with commit a8e5e5146ad0 ("arm64:
> > > mte: Avoid setting PG_mte_tagged if no tags cleared or restored").
> >
> > This commit seems part of -rc1, so I guess the patches directly apply
> > on top of that tag?
> 
> Yes, sorry, this also applies cleanly to -rc1.
> 
> > > I now have Reviewed-by for all patches except for the last one, which adds
> > > the documentation. Thanks for the reviews so far, and please take a look!
> >
> > I'd really like the MM folks (list now cc'd) to look at the relevant
> > patches (1 and 5) and ack them before I take this.
> 
> Okay, here are the lore links for the convenience of the MM folks:
> https://lore.kernel.org/all/20221104011041.290951-2-pcc@google.com/
> https://lore.kernel.org/all/20221104011041.290951-6-pcc@google.com/

I have not seen any Ack from the MM folks so far, and we're really
running out of runway for this merge window.

Short of someone shouting now, I'll take the series into the kvmarm
tree early next week.

Thanks,


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