Re: [PATCH v2 2/7] KVM: arm64: Add FEAT_TLBIRANGE support

Raghavendra Rao Ananta
2023-04-03T17:26:18Z

Hi Oliver,

On Wed, Mar 29, 2023 at 6:19 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> On Mon, Feb 06, 2023 at 05:23:35PM +0000, Raghavendra Rao Ananta wrote:
> > Define a generic function __kvm_tlb_flush_range() to
> > invalidate the TLBs over a range of addresses. The
> > implementation accepts 'op' as a generic TLBI operation.
> > Upcoming patches will use this to implement IPA based
> > TLB invalidations (ipas2e1is).
> >
> > If the system doesn't support FEAT_TLBIRANGE, the
> > implementation falls back to flushing the pages one by one
> > for the range supplied.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> > ---
> > arch/arm64/include/asm/kvm_asm.h | 18 ++++++++++++++++++
> > 1 file changed, 18 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > index 43c3bc0f9544d..995ff048e8851 100644
> > --- a/arch/arm64/include/asm/kvm_asm.h
> > +++ b/arch/arm64/include/asm/kvm_asm.h
> > @@ -221,6 +221,24 @@ DECLARE_KVM_NVHE_SYM(__per_cpu_end);
> > DECLARE_KVM_HYP_SYM(__bp_harden_hyp_vecs);
> > #define __bp_harden_hyp_vecs CHOOSE_HYP_SYM(__bp_harden_hyp_vecs)
> >
> > +#define __kvm_tlb_flush_range(op, mmu, start, end, level, tlb_level) do { \
> > + unsigned long pages, stride; \
> > + \
> > + stride = kvm_granule_size(level); \
>
> Hmm... There's a rather subtle and annoying complication here that I
> don't believe is handled.
>
> Similar to what I said in the last spin of the series, there is no
> guarantee that a range of IPAs is mapped at the exact same level
> throughout. Dirty logging and memslots that aren't hugepage aligned
> could lead to a mix of mapping levels being used within a range of the
> IPA space.
>
Unlike the comment on v1, the level/stride here is used to jump the
addresses in case the system doesn't support TLBIRANGE. The TTL hint
is 0.
That being said, do you think we can always assume the least possible
stride (say, 4k) and hardcode it?
With respect to alignment, since the function is only called while
breaking the table PTE, do you think it'll still be a problem even if
we go with the least granularity stride?

> > + start = round_down(start, stride); \
> > + end = round_up(end, stride); \
> > + pages = (end - start) >> PAGE_SHIFT; \
> > + \
> > + if ((!system_supports_tlb_range() && \
> > + (end - start) >= (MAX_TLBI_OPS * stride)) || \
>
> Doesn't checking for TLBIRANGE above eliminate the need to test against
> MAX_TLBI_OPS?
>
Derived from __flush_tlb_range(), I think the condition is used to
just flush everything if the range is too large to iterate and flush
when the system doesn't support TLBIRANGE. Probably to prevent
soft-lockups?

Thank you.
Raghavendra
> > + pages >= MAX_TLBI_RANGE_PAGES) { \
> > + __kvm_tlb_flush_vmid(mmu); \
> > + break; \
> > + } \
> > + \
> > + __flush_tlb_range_op(op, start, pages, stride, 0, tlb_level, false); \
> > +} while (0)
> > +
> > extern void __kvm_flush_vm_context(void);
> > extern void __kvm_flush_cpu_context(struct kvm_s2_mmu *mmu);
> > extern void __kvm_tlb_flush_vmid_ipa(struct kvm_s2_mmu *mmu, phys_addr_t ipa,
> > --
> > 2.39.1.519.gcb327c4b5f-goog
> >
> >
>
> --
> Thanks,
> Oliver