Re: [PATCH] KVM: x86: Reduce retpoline performance impact in slot_handle_level_range()

From: Sironi, Filippo
Date: Fri Feb 02 2018 - 10:44:38 EST



> On 2. Feb 2018, at 15:59, David Woodhouse <dwmw@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
> 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


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