Re: [PATCH v4 3/6] KVM: arm64: Implement kvm_arch_flush_remote_tlbs_range()

[Raghavendra Rao Ananta]

On Wed, May 31, 2023 at 1:46 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Tue, 30 May 2023 22:22:23 +0100,
> Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> >
> > On Mon, May 29, 2023 at 7:00 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > On Fri, 19 May 2023 01:52:28 +0100,
> > > Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> > > >
> > > > Implement kvm_arch_flush_remote_tlbs_range() for arm64
> > > > to invalidate the given range in the TLB.
> > > >
> > > > Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> > > > ---
> > > >  arch/arm64/include/asm/kvm_host.h |  3 +++
> > > >  arch/arm64/kvm/hyp/nvhe/tlb.c     |  4 +---
> > > >  arch/arm64/kvm/mmu.c              | 11 +++++++++++
> > > >  3 files changed, 15 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > > index 81ab41b84f436..343fb530eea9c 100644
> > > > --- a/arch/arm64/include/asm/kvm_host.h
> > > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > > @@ -1081,6 +1081,9 @@ struct kvm *kvm_arch_alloc_vm(void);
> > > >  #define __KVM_HAVE_ARCH_FLUSH_REMOTE_TLBS
> > > >  int kvm_arch_flush_remote_tlbs(struct kvm *kvm);
> > > >
> > > > +#define __KVM_HAVE_ARCH_FLUSH_REMOTE_TLBS_RANGE
> > > > +int kvm_arch_flush_remote_tlbs_range(struct kvm *kvm, gfn_t start_gfn, u64 pages);
> > > > +
> > > >  static inline bool kvm_vm_is_protected(struct kvm *kvm)
> > > >  {
> > > >       return false;
> > > > diff --git a/arch/arm64/kvm/hyp/nvhe/tlb.c b/arch/arm64/kvm/hyp/nvhe/tlb.c
> > > > index d4ea549c4b5c4..d2c7c1bc6d441 100644
> > > > --- a/arch/arm64/kvm/hyp/nvhe/tlb.c
> > > > +++ b/arch/arm64/kvm/hyp/nvhe/tlb.c
> > > > @@ -150,10 +150,8 @@ void __kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu,
> > > >               return;
> > > >       }
> > > >
> > > > -     dsb(ishst);
> > > > -
> > > >       /* Switch to requested VMID */
> > > > -     __tlb_switch_to_guest(mmu, &cxt);
> > > > +     __tlb_switch_to_guest(mmu, &cxt, false);
> > >
> > > This hunk is in the wrong patch, isn't it?
> > >
> > Ah, you are right. It should be part of the previous patch. I think I
> > introduced it accidentally when I rebased the series. I'll remove it
> > in the next spin.
> >
> >
> > > >
> > > >       __flush_tlb_range_op(ipas2e1is, start, pages, stride, 0, 0, false);
> > > >
> > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > > > index d0a0d3dca9316..e3673b4c10292 100644
> > > > --- a/arch/arm64/kvm/mmu.c
> > > > +++ b/arch/arm64/kvm/mmu.c
> > > > @@ -92,6 +92,17 @@ int kvm_arch_flush_remote_tlbs(struct kvm *kvm)
> > > >       return 0;
> > > >  }
> > > >
> > > > +int kvm_arch_flush_remote_tlbs_range(struct kvm *kvm, gfn_t start_gfn, u64 pages)
> > > > +{
> > > > +     phys_addr_t start, end;
> > > > +
> > > > +     start = start_gfn << PAGE_SHIFT;
> > > > +     end = (start_gfn + pages) << PAGE_SHIFT;
> > > > +
> > > > +     kvm_call_hyp(__kvm_tlb_flush_vmid_range, &kvm->arch.mmu, start, end);
> > >
> > > So that's the point that I think is not right. It is the MMU code that
> > > should drive the invalidation method, and not the HYP code. The HYP
> > > code should be as dumb as possible, and the logic should be kept in
> > > the MMU code.
> > >
> > > So when a range invalidation is forwarded to HYP, it's a *valid* range
> > > invalidation. not something that can fallback to VMID-wide invalidation.
> > >
> > I'm guessing that you are referring to patch-2. Do you recommend
> > moving the 'pages >= MAX_TLBI_RANGE_PAGES' logic here and simply
> > return an error? How about for the other check:
> > system_supports_tlb_range()?
> > The idea was for __kvm_tlb_flush_vmid_range() to also implement a
> > fallback mechanism in case the system doesn't support the range-based
> > instructions. But if we end up calling __kvm_tlb_flush_vmid_range()
> > from multiple cases, we'd end up duplicating the checks. WDYT?
>
> My take is that there should be a single helper deciding to issue
> either a number of range-based TLBIs depending on start/end, or a
> single VMID-based TLBI. Having multiple calling sites is not a
> problem, and even if that code gets duplicated, big deal.
>
Hypothetically, if I move the check to this patch and return an error
if this situation occurs, since I'm dependending on David's common MMU
code [1], kvm_main.c would end of calling kvm_flush_remote_tlbs() and
we'd be doing a VMID-based TLBI.
One idea would be to issue a WARN_ON() and return 0 so that we don't
issue any TLBIs. Thoughts?

> But a hypercall that falls back to global invalidation based on a
> range evaluation error (more than MAX_TLBI_RANGE_PAGES) is papering
> over a latent bug.
>
If I understand this correctly, MAX_TLBI_RANGE_PAGES is specifically
the capacity of the range-based instructions itself, isn't it? Is it
incorrect for the caller to request a higher range be invalidated even
on systems that do not support these instructions? Probably that's why
__flush_tlb_range() falls back to a global flush when the range
request is exceeded?

Thank you.
Raghavendra

[1]:  https://lore.kernel.org/linux-arm-kernel/20230126184025.2294823-7-dmatlack@xxxxxxxxxx/


> There should be no logic whatsoever in any of the two tlb.c files.
> Only a switch to the correct context, and the requested invalidation,
> which *must* be architecturally correct.
>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.