Re: [RFC PATCH 16/17] KVM: arm64: Enable parallel stage 2 MMU faults

[Oliver Upton <oupton@google.com>]

On Thu, Apr 21, 2022 at 09:35:27AM -0700, Ben Gardon wrote:
> On Fri, Apr 15, 2022 at 2:59 PM Oliver Upton <oupton@google.com> wrote:
> >
> > Voila! Since the map walkers are able to work in parallel there is no
> > need to take the write lock on a stage 2 memory abort. Relax locking
> > on map operations and cross fingers we got it right.
> 
> Might be worth a healthy sprinkle of lockdep on the functions taking
> "shared" as an argument, just to make sure the wrong value isn't going
> down a callstack you didn't expect.

If we're going to go this route we might need to just punch a pointer
to the vCPU through to the stage 2 table walker. All of this plumbing is
built around the idea that there are multiple tables to manage and
needn't be in the context of a vCPU/VM, which is why I went the WARN()
route instead of better lockdep assertions.

> >
> > Signed-off-by: Oliver Upton <oupton@google.com>
> > ---
> >  arch/arm64/kvm/mmu.c | 21 +++------------------
> >  1 file changed, 3 insertions(+), 18 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 63cf18cdb978..2881051c3743 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1127,7 +1127,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >         gfn_t gfn;
> >         kvm_pfn_t pfn;
> >         bool logging_active = memslot_is_logging(memslot);
> > -       bool use_read_lock = false;
> >         unsigned long fault_level = kvm_vcpu_trap_get_fault_level(vcpu);
> >         unsigned long vma_pagesize, fault_granule;
> >         enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> > @@ -1162,8 +1161,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >         if (logging_active) {
> >                 force_pte = true;
> >                 vma_shift = PAGE_SHIFT;
> > -               use_read_lock = (fault_status == FSC_PERM && write_fault &&
> > -                                fault_granule == PAGE_SIZE);
> >         } else {
> >                 vma_shift = get_vma_page_shift(vma, hva);
> >         }
> > @@ -1267,15 +1264,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >         if (exec_fault && device)
> >                 return -ENOEXEC;
> >
> > -       /*
> > -        * To reduce MMU contentions and enhance concurrency during dirty
> > -        * logging dirty logging, only acquire read lock for permission
> > -        * relaxation.
> > -        */
> > -       if (use_read_lock)
> > -               read_lock(&kvm->mmu_lock);
> > -       else
> > -               write_lock(&kvm->mmu_lock);
> > +       read_lock(&kvm->mmu_lock);
> > +
> 
> Ugh, I which we could get rid of the analogous ugly block on x86.

Maybe we could fold it in to a MMU macro in the arch-generic scope?
Conditional locking is smelly, I was very pleased to delete these lines :)

--
Thanks,
Oliver
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