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SubjectRe: [PATCH] KVM: x86: Reduce retpoline performance impact in slot_handle_level_range()
> > On 2. Feb 2018, at 15:59, David Woodhouse <dwmw@amazon.co.uk> wrote:
> > With retpoline, tight loops of "call this function for every XXX" are
> > very much pessimised by taking a prediction miss *every* time.
> >
> > This one showed up very high in our early testing, and it only has five
> > things it'll ever call so make it take an 'op' enum instead of a
> > function pointer and let's see how that works out...
> >
> > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

What about __always_inline instead?

Thanks,

Paolo

> > ---
> > Not sure I like this. Better suggestions welcomed...
> >
> > In the general case, we have a few things we can do with the calls that
> > retpoline turns into bottlenecks. This is one of them.
> >
> > Another option, if there are just one or two "likely" functions, is
> > something along the lines of
> >
> > if (func == likelyfunc)
> > likelyfunc()
> > else
> > (*func)(); // GCC does retpoline for this
> >
> > For things like kvm_x86_ops we really could just turn *all* of those
> > into direct calls at runtime, like pvops does.
> >
> > There are some which land somewhere in the middle, like the default
> > dma_ops. We probably want something like the 'likelyfunc' version
> > above, except that we *also* want to flip the likelyfunc between the
> > Intel and AMD IOMMU ops functions, at early boot. I'll see what I can
> > come up with...
> >
> > arch/x86/kvm/mmu.c | 72
> > ++++++++++++++++++++++++++++++++++++------------------
> > 1 file changed, 48 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index 2b8eb4d..44f9de7 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -5055,12 +5055,21 @@ void kvm_mmu_uninit_vm(struct kvm *kvm)
> > }
> >
> > /* The return value indicates if tlb flush on all vcpus is needed. */
> > -typedef bool (*slot_level_handler) (struct kvm *kvm, struct kvm_rmap_head
> > *rmap_head);
> > +enum slot_handler_op {
> > + SLOT_RMAP_CLEAR_DIRTY,
> > + SLOT_RMAP_SET_DIRTY,
> > + SLOT_RMAP_WRITE_PROTECT,
> > + SLOT_ZAP_RMAPP,
> > + SLOT_ZAP_COLLAPSIBLE_SPTE,
> > +};
> > +
> > +static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> > + struct kvm_rmap_head *rmap_head);
> >
> > /* The caller should hold mmu-lock before calling this function. */
> > static bool
> > slot_handle_level_range(struct kvm *kvm, struct kvm_memory_slot *memslot,
> > - slot_level_handler fn, int start_level, int end_level,
> > + enum slot_handler_op op, int start_level, int end_level,
> > gfn_t start_gfn, gfn_t end_gfn, bool lock_flush_tlb)
> > {
> > struct slot_rmap_walk_iterator iterator;
> > @@ -5068,8 +5077,29 @@ slot_handle_level_range(struct kvm *kvm, struct
> > kvm_memory_slot *memslot,
> >
> > for_each_slot_rmap_range(memslot, start_level, end_level, start_gfn,
> > end_gfn, &iterator) {
> > - if (iterator.rmap)
> > - flush |= fn(kvm, iterator.rmap);
> > + if (iterator.rmap) {
> > + switch (op) {
> > + case SLOT_RMAP_CLEAR_DIRTY:
> > + flush |= __rmap_clear_dirty(kvm, iterator.rmap);
> > + break;
> > +
> > + case SLOT_RMAP_SET_DIRTY:
> > + flush |= __rmap_set_dirty(kvm, iterator.rmap);
> > + break;
> > +
> > + case SLOT_RMAP_WRITE_PROTECT:
> > + flush |= __rmap_write_protect(kvm, iterator.rmap, false);
> > + break;
> > +
> > + case SLOT_ZAP_RMAPP:
> > + flush |= kvm_zap_rmapp(kvm, iterator.rmap);
> > + break;
> > +
> > + case SLOT_ZAP_COLLAPSIBLE_SPTE:
> > + flush |= kvm_mmu_zap_collapsible_spte(kvm, iterator.rmap);
> > + break;
> > + }
> > + }
> >
> > if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
> > if (flush && lock_flush_tlb) {
> > @@ -5090,10 +5120,10 @@ slot_handle_level_range(struct kvm *kvm, struct
> > kvm_memory_slot *memslot,
> >
> > static bool
> > slot_handle_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
> > - slot_level_handler fn, int start_level, int end_level,
> > + enum slot_handler_op op, int start_level, int end_level,
> > bool lock_flush_tlb)
> > {
> > - return slot_handle_level_range(kvm, memslot, fn, start_level,
> > + return slot_handle_level_range(kvm, memslot, op, start_level,
> > end_level, memslot->base_gfn,
> > memslot->base_gfn + memslot->npages - 1,
> > lock_flush_tlb);
> > @@ -5101,25 +5131,25 @@ slot_handle_level(struct kvm *kvm, struct
> > kvm_memory_slot *memslot,
> >
> > static bool
> > slot_handle_all_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
> > - slot_level_handler fn, bool lock_flush_tlb)
> > + enum slot_handler_op op, bool lock_flush_tlb)
> > {
> > - return slot_handle_level(kvm, memslot, fn, PT_PAGE_TABLE_LEVEL,
> > + return slot_handle_level(kvm, memslot, op, PT_PAGE_TABLE_LEVEL,
> > PT_MAX_HUGEPAGE_LEVEL, lock_flush_tlb);
> > }
> >
> > static bool
> > slot_handle_large_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
> > - slot_level_handler fn, bool lock_flush_tlb)
> > + enum slot_handler_op op, bool lock_flush_tlb)
> > {
> > - return slot_handle_level(kvm, memslot, fn, PT_PAGE_TABLE_LEVEL + 1,
> > + return slot_handle_level(kvm, memslot, op, PT_PAGE_TABLE_LEVEL + 1,
> > PT_MAX_HUGEPAGE_LEVEL, lock_flush_tlb);
> > }
> >
> > static bool
> > slot_handle_leaf(struct kvm *kvm, struct kvm_memory_slot *memslot,
> > - slot_level_handler fn, bool lock_flush_tlb)
> > + enum slot_handler_op op, bool lock_flush_tlb)
> > {
> > - return slot_handle_level(kvm, memslot, fn, PT_PAGE_TABLE_LEVEL,
> > + return slot_handle_level(kvm, memslot, op, PT_PAGE_TABLE_LEVEL,
> > PT_PAGE_TABLE_LEVEL, lock_flush_tlb);
> > }
> >
> > @@ -5140,7 +5170,7 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t
> > gfn_start, gfn_t gfn_end)
> > if (start >= end)
> > continue;
> >
> > - slot_handle_level_range(kvm, memslot, kvm_zap_rmapp,
> > + slot_handle_level_range(kvm, memslot, SLOT_ZAP_RMAPP,
> > PT_PAGE_TABLE_LEVEL, PT_MAX_HUGEPAGE_LEVEL,
> > start, end - 1, true);
> > }
> > @@ -5149,19 +5179,13 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t
> > gfn_start, gfn_t gfn_end)
> > spin_unlock(&kvm->mmu_lock);
> > }
> >
> > -static bool slot_rmap_write_protect(struct kvm *kvm,
> > - struct kvm_rmap_head *rmap_head)
> > -{
> > - return __rmap_write_protect(kvm, rmap_head, false);
> > -}
> > -
> > void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
> > struct kvm_memory_slot *memslot)
> > {
> > bool flush;
> >
> > spin_lock(&kvm->mmu_lock);
> > - flush = slot_handle_all_level(kvm, memslot, slot_rmap_write_protect,
> > + flush = slot_handle_all_level(kvm, memslot, SLOT_RMAP_WRITE_PROTECT,
> > false);
> > spin_unlock(&kvm->mmu_lock);
> >
> > @@ -5226,7 +5250,7 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
> > /* FIXME: const-ify all uses of struct kvm_memory_slot. */
> > spin_lock(&kvm->mmu_lock);
> > slot_handle_leaf(kvm, (struct kvm_memory_slot *)memslot,
> > - kvm_mmu_zap_collapsible_spte, true);
> > + SLOT_ZAP_COLLAPSIBLE_SPTE, true);
> > spin_unlock(&kvm->mmu_lock);
> > }
> >
> > @@ -5236,7 +5260,7 @@ void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
> > bool flush;
> >
> > spin_lock(&kvm->mmu_lock);
> > - flush = slot_handle_leaf(kvm, memslot, __rmap_clear_dirty, false);
> > + flush = slot_handle_leaf(kvm, memslot, SLOT_RMAP_CLEAR_DIRTY, false);
> > spin_unlock(&kvm->mmu_lock);
> >
> > lockdep_assert_held(&kvm->slots_lock);
> > @@ -5258,7 +5282,7 @@ void
> > kvm_mmu_slot_largepage_remove_write_access(struct kvm *kvm,
> > bool flush;
> >
> > spin_lock(&kvm->mmu_lock);
> > - flush = slot_handle_large_level(kvm, memslot, slot_rmap_write_protect,
> > + flush = slot_handle_large_level(kvm, memslot, SLOT_RMAP_WRITE_PROTECT,
> > false);
> > spin_unlock(&kvm->mmu_lock);
> >
> > @@ -5276,7 +5300,7 @@ void kvm_mmu_slot_set_dirty(struct kvm *kvm,
> > bool flush;
> >
> > spin_lock(&kvm->mmu_lock);
> > - flush = slot_handle_all_level(kvm, memslot, __rmap_set_dirty, false);
> > + flush = slot_handle_all_level(kvm, memslot, SLOT_RMAP_SET_DIRTY, false);
> > spin_unlock(&kvm->mmu_lock);
> >
> > lockdep_assert_held(&kvm->slots_lock);
> > --
> > 2.7.4
> >
>
> Let's add more context.
>
> vmx_slot_disable_log_dirty() was already one of the bottlenecks on instance
> launch
> (at least with our setup). With retpoline, it became horribly slow (like
> twice as
> slow).
>
> Up to know, we're using a ugly workaround that works for us but of course
> isn't
> acceptable in the long run. I'm going to explore the issue further earlier
> next
> week.
>
> Filippo
>
>
> Amazon Development Center Germany GmbH
> Berlin - Dresden - Aachen
> main office: Krausenstr. 38, 10117 Berlin
> Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
> Ust-ID: DE289237879
> Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
>
>

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