diff options
| author | Sean Christopherson <seanjc@google.com> | 2021-03-30 17:49:42 -0700 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2021-04-17 08:30:57 -0400 |
| commit | 8f8f52a45d928d638c7ffbd081de85c692b28964 (patch) | |
| tree | 2bf767d55be962c246f9d715aca819ce6b395aca /arch/x86 | |
| parent | 6d9aafb96d5f665d038229e7561388c38430b3a8 (diff) | |
| download | linux-8f8f52a45d928d638c7ffbd081de85c692b28964.tar.gz | |
KVM: x86/mmu: Simplify code for aging SPTEs in TDP MMU
Use a basic NOT+AND sequence to clear the Accessed bit in TDP MMU SPTEs,
as opposed to the fancy ffs()+clear_bit() logic that was copied from the
legacy MMU. The legacy MMU uses clear_bit() because it is operating on
the SPTE itself, i.e. clearing needs to be atomic. The TDP MMU operates
on a local variable that it later writes to the SPTE, and so doesn't need
to be atomic or even resident in memory.
Opportunistically drop unnecessary initialization of new_spte, it's
guaranteed to be written before being accessed.
Using NOT+AND instead of ffs()+clear_bit() reduces the sequence from:
0x0000000000058be6 <+134>: test %rax,%rax
0x0000000000058be9 <+137>: je 0x58bf4 <age_gfn_range+148>
0x0000000000058beb <+139>: test %rax,%rdi
0x0000000000058bee <+142>: je 0x58cdc <age_gfn_range+380>
0x0000000000058bf4 <+148>: mov %rdi,0x8(%rsp)
0x0000000000058bf9 <+153>: mov $0xffffffff,%edx
0x0000000000058bfe <+158>: bsf %eax,%edx
0x0000000000058c01 <+161>: movslq %edx,%rdx
0x0000000000058c04 <+164>: lock btr %rdx,0x8(%rsp)
0x0000000000058c0b <+171>: mov 0x8(%rsp),%r15
to:
0x0000000000058bdd <+125>: test %rax,%rax
0x0000000000058be0 <+128>: je 0x58beb <age_gfn_range+139>
0x0000000000058be2 <+130>: test %rax,%r8
0x0000000000058be5 <+133>: je 0x58cc0 <age_gfn_range+352>
0x0000000000058beb <+139>: not %rax
0x0000000000058bee <+142>: and %r8,%rax
0x0000000000058bf1 <+145>: mov %rax,%r15
thus eliminating several memory accesses, including a locked access.
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210331004942.2444916-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'arch/x86')
| -rw-r--r-- | arch/x86/kvm/mmu/tdp_mmu.c | 5 |
1 files changed, 2 insertions, 3 deletions
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index ad688e75ce88..ccf0d774a181 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.c +++ b/arch/x86/kvm/mmu/tdp_mmu.c @@ -950,7 +950,7 @@ static int age_gfn_range(struct kvm *kvm, struct kvm_memory_slot *slot, { struct tdp_iter iter; int young = 0; - u64 new_spte = 0; + u64 new_spte; rcu_read_lock(); @@ -965,8 +965,7 @@ static int age_gfn_range(struct kvm *kvm, struct kvm_memory_slot *slot, new_spte = iter.old_spte; if (spte_ad_enabled(new_spte)) { - clear_bit((ffs(shadow_accessed_mask) - 1), - (unsigned long *)&new_spte); + new_spte &= ~shadow_accessed_mask; } else { /* * Capture the dirty status of the page, so that it doesn't get |