Re: [PATCH 08/14] KVM: arm64: Protect page table traversal with RCU

Oliver Upton
2022-09-09T09:56:52Z

On Wed, Sep 07, 2022 at 02:47:08PM -0700, David Matlack wrote:
> On Tue, Aug 30, 2022 at 07:41:26PM +0000, Oliver Upton wrote:
> > The use of RCU is necessary to change the paging structures in parallel.
> > Acquire and release an RCU read lock when traversing the page tables.
> >
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> > ---
> > arch/arm64/include/asm/kvm_pgtable.h | 19 ++++++++++++++++++-
> > arch/arm64/kvm/hyp/pgtable.c | 7 ++++++-
> > 2 files changed, 24 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index 78fbb7be1af6..7d2de0a98ccb 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -578,9 +578,26 @@ enum kvm_pgtable_prot kvm_pgtable_stage2_pte_prot(kvm_pte_t pte);
> > */
> > enum kvm_pgtable_prot kvm_pgtable_hyp_pte_prot(kvm_pte_t pte);
> >
> > +#if defined(__KVM_NVHE_HYPERVISOR___)
> > +
>
> Future readers will wonder why NVHE stubs out RCU support and how that
> is even correct. Some comments here would be useful explain it.

Good point.

> > +static inline void kvm_pgtable_walk_begin(void) {}
> > +static inline void kvm_pgtable_walk_end(void) {}
> > +
> > +#define kvm_dereference_ptep rcu_dereference_raw
>
> How does NVHE have access rcu_dereference_raw()?

rcu_dereference_raw() is inlined and simply recasts the pointer into the
kernel address space.

Perhaps it is less confusing to template this on kvm_pte_read() to avoid
polluting nVHE with an otherwise benign reference to RCU.

> > +
> > +#else /* !defined(__KVM_NVHE_HYPERVISOR__) */
> > +
> > +#define kvm_pgtable_walk_begin rcu_read_lock
> > +#define kvm_pgtable_walk_end rcu_read_unlock
> > +#define kvm_dereference_ptep rcu_dereference
> > +
> > +#endif /* defined(__KVM_NVHE_HYPERVISOR__) */
> > +
> > static inline kvm_pte_t kvm_pte_read(kvm_pte_t *ptep)
> > {
> > - return READ_ONCE(*ptep);
> > + kvm_pte_t __rcu *p = (kvm_pte_t __rcu *)ptep;
> > +
> > + return READ_ONCE(*kvm_dereference_ptep(p));
>
> What about all the other places where page table memory is accessed?
>
> If RCU is going to be used to protect page table memory, then all
> accesses have to go under an RCU critical section. This means that page
> table memory should only be accessed through __rcu annotated pointers
> and dereferenced with rcu_dereference().

Let me play around with this a bit, as the annoying part is trying to
sprinkle in RCU annotations w/o messing with nVHE.

--
Thanks,
Oliver