Commit 8f5c44f9 authored Feb 08, 2021 by Maciej S. Szmigiero Committed by Paolo Bonzini Feb 09, 2021

KVM: x86/mmu: Make HVA handler retpoline-friendly



When retpolines are enabled they have high overhead in the inner loop
inside kvm_handle_hva_range() that iterates over the provided memory area.

Let's mark this function and its TDP MMU equivalent __always_inline so
compiler will be able to change the call to the actual handler function
inside each of them into a direct one.

This significantly improves performance on the unmap test on the existing
kernel memslot code (tested on a Xeon 8167M machine):
30 slots in use:
Test       Before   After     Improvement
Unmap      0.0353s  0.0334s   5%
Unmap 2M   0.00104s 0.000407s 61%

509 slots in use:
Test       Before   After     Improvement
Unmap      0.0742s  0.0740s   None
Unmap 2M   0.00221s 0.00159s  28%

Looks like having an indirect call in these functions (and, so, a
retpoline) might have interfered with unrolling of the whole loop in the
CPU.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <732d3fe9eb68aa08402a638ab0309199fa89ae56.1612810129.git.maciej.szmigiero@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent b9ce0f86

arch/x86/kvm/mmu/mmu.c

+11 −10

Original line number	Diff line number	Diff line
		@@ -1457,7 +1457,8 @@ static void slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
		slot_rmap_walk_okay(_iter_); \
		slot_rmap_walk_next(_iter_))

		static int kvm_handle_hva_range(struct kvm *kvm,
		static __always_inline int
		kvm_handle_hva_range(struct kvm *kvm,
		unsigned long start,
		unsigned long end,
		unsigned long data,

arch/x86/kvm/mmu/tdp_mmu.c

+11 −5

Original line number	Diff line number	Diff line
		@@ -879,11 +879,17 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
		return ret;
		}

		static int kvm_tdp_mmu_handle_hva_range(struct kvm *kvm, unsigned long start,
		unsigned long end, unsigned long data,
		int (handler)(struct kvm kvm, struct kvm_memory_slot *slot,
		struct kvm_mmu_page *root, gfn_t start,
		gfn_t end, unsigned long data))
		static __always_inline int
		kvm_tdp_mmu_handle_hva_range(struct kvm *kvm,
		unsigned long start,
		unsigned long end,
		unsigned long data,
		int (handler)(struct kvm kvm,
		struct kvm_memory_slot *slot,
		struct kvm_mmu_page *root,
		gfn_t start,
		gfn_t end,
		unsigned long data))
		{
		struct kvm_memslots *slots;
		struct kvm_memory_slot *memslot;