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Subject[RFC PATCH 2/4] KVM: x86: Reduce retpoline performance impact in slot_handle_level_range()
Date
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.

By marking the iterator slot_handle_…() functions always_inline, we can
ensure that the indirect function call can be optimised away into a
direct call and it actually generates slightly smaller code because
some of the other conditionals can get optimised away too.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
arch/x86/kvm/mmu.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 2b8eb4d..cc83bdc 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5058,7 +5058,7 @@ void kvm_mmu_uninit_vm(struct kvm *kvm)
typedef bool (*slot_level_handler) (struct kvm *kvm, struct kvm_rmap_head *rmap_head);

/* The caller should hold mmu-lock before calling this function. */
-static bool
+static __always_inline bool
slot_handle_level_range(struct kvm *kvm, struct kvm_memory_slot *memslot,
slot_level_handler fn, int start_level, int end_level,
gfn_t start_gfn, gfn_t end_gfn, bool lock_flush_tlb)
@@ -5088,7 +5088,7 @@ slot_handle_level_range(struct kvm *kvm, struct kvm_memory_slot *memslot,
return flush;
}

-static bool
+static __always_inline bool
slot_handle_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
slot_level_handler fn, int start_level, int end_level,
bool lock_flush_tlb)
@@ -5099,7 +5099,7 @@ slot_handle_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
lock_flush_tlb);
}

-static bool
+static __always_inline bool
slot_handle_all_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
slot_level_handler fn, bool lock_flush_tlb)
{
@@ -5107,7 +5107,7 @@ slot_handle_all_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
PT_MAX_HUGEPAGE_LEVEL, lock_flush_tlb);
}

-static bool
+static __always_inline bool
slot_handle_large_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
slot_level_handler fn, bool lock_flush_tlb)
{
@@ -5115,7 +5115,7 @@ slot_handle_large_level(struct kvm *kvm, struct kvm_memory_slot *memslot,
PT_MAX_HUGEPAGE_LEVEL, lock_flush_tlb);
}

-static bool
+static __always_inline bool
slot_handle_leaf(struct kvm *kvm, struct kvm_memory_slot *memslot,
slot_level_handler fn, bool lock_flush_tlb)
{
--
2.7.4
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