Re: [PATCH v3 06/15] KVM: arm64: Implement kvm_pgtable_hyp_unmap() at EL2

Andrew Walbran
2021-12-08T14:42:42Z

On Wed, 8 Dec 2021 at 09:51, Quentin Perret <qperret@google.com> wrote:
>
> Hi Andrew,
>
> On Tuesday 07 Dec 2021 at 14:47:14 (+0000), Andrew Walbran wrote:
> > On Wed, 1 Dec 2021 at 17:04, 'Quentin Perret' via kernel-team
> > <kernel-team@android.com> wrote:
> > >
> > > From: Will Deacon <will@kernel.org>
> > >
> > > Implement kvm_pgtable_hyp_unmap() which can be used to remove hypervisor
> > > stage-1 mappings at EL2.
> > >
> > > Signed-off-by: Will Deacon <will@kernel.org>
> > > Signed-off-by: Quentin Perret <qperret@google.com>
> > > ---
> > > arch/arm64/include/asm/kvm_pgtable.h | 21 ++++++++++
> > > arch/arm64/kvm/hyp/pgtable.c | 63 ++++++++++++++++++++++++++++
> > > 2 files changed, 84 insertions(+)
> > >
> > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > > index 027783829584..9d076f36401d 100644
> > > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > > @@ -251,6 +251,27 @@ void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt);
> > > int kvm_pgtable_hyp_map(struct kvm_pgtable *pgt, u64 addr, u64 size, u64 phys,
> > > enum kvm_pgtable_prot prot);
> > >
> > > +/**
> > > + * kvm_pgtable_hyp_unmap() - Remove a mapping from a hypervisor stage-1 page-table.
> > > + * @pgt: Page-table structure initialised by kvm_pgtable_hyp_init().
> > > + * @addr: Virtual address from which to remove the mapping.
> > > + * @size: Size of the mapping.
> > > + *
> > > + * The offset of @addr within a page is ignored, @size is rounded-up to
> > > + * the next page boundary and @phys is rounded-down to the previous page
> > > + * boundary.
> > > + *
> > > + * TLB invalidation is performed for each page-table entry cleared during the
> > > + * unmapping operation and the reference count for the page-table page
> > > + * containing the cleared entry is decremented, with unreferenced pages being
> > > + * freed. The unmapping operation will stop early if it encounters either an
> > > + * invalid page-table entry or a valid block mapping which maps beyond the range
> > > + * being unmapped.
> >
> > How is the caller expected to break up the block mapping? Why not
> > handle that within this function?
>
> We don't really use block mappings for the hyp stage-1, since pretty
> much forever (see the loop in pkvm_create_mappings_locked() for ex), so
> handling it here would be somewhat unnecessary complexity. Handling this
> in the pgtable code itself (which I assume would mean proactively
> re-mapping the rest of the range with page-granularity mappings or
> something along those lines) is tricky because of BBM and concurrency,
> so I'd rather avoid handling same-level aborts at EL2 and all that mess
> unless we have a good reason. Is there a use-case where you think that'd
> be needed?

Aha, I didn't realise that block mappings, but it makes sense in that
case. How about adding a note to the function comment here explaining
that reasoning? Otherwise it sounds like the caller has to handle it
somehow.