* [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups
@ 2021-03-31 0:49 Sean Christopherson
2021-03-31 0:49 ` [PATCH 1/2] KVM: x86/mmu: Remove spurious clearing of dirty bit from TDP MMU SPTE Sean Christopherson
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: Sean Christopherson @ 2021-03-31 0:49 UTC (permalink / raw)
To: Paolo Bonzini
Cc: Sean Christopherson, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
Joerg Roedel, kvm, linux-kernel, Ben Gardon
Two minor fixes/cleanups for the TDP MMU, found by inspection.
Sean Christopherson (2):
KVM: x86/mmu: Remove spurious clearing of dirty bit from TDP MMU SPTE
KVM: x86/mmu: Simplify code for aging SPTEs in TDP MMU
arch/x86/kvm/mmu/tdp_mmu.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
--
2.31.0.291.g576ba9dcdaf-goog
^ permalink raw reply [flat|nested] 4+ messages in thread
* [PATCH 1/2] KVM: x86/mmu: Remove spurious clearing of dirty bit from TDP MMU SPTE
2021-03-31 0:49 [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups Sean Christopherson
@ 2021-03-31 0:49 ` Sean Christopherson
2021-03-31 0:49 ` [PATCH 2/2] KVM: x86/mmu: Simplify code for aging SPTEs in TDP MMU Sean Christopherson
2021-03-31 9:36 ` [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups Paolo Bonzini
2 siblings, 0 replies; 4+ messages in thread
From: Sean Christopherson @ 2021-03-31 0:49 UTC (permalink / raw)
To: Paolo Bonzini
Cc: Sean Christopherson, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
Joerg Roedel, kvm, linux-kernel, Ben Gardon
Don't clear the dirty bit when aging a TDP MMU SPTE (in response to a MMU
notifier event). Prematurely clearing the dirty bit could cause spurious
PML updates if aging a page happened to coincide with dirty logging.
Note, tdp_mmu_set_spte_no_acc_track() flows into __handle_changed_spte(),
so the host PFN will be marked dirty, i.e. there is no potential for data
corruption.
Fixes: a6a0b05da9f3 ("kvm: x86/mmu: Support dirty logging for the TDP MMU")
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/tdp_mmu.c | 1 -
1 file changed, 1 deletion(-)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index f0c99fa04ef2..724088bea4b0 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -978,7 +978,6 @@ static int age_gfn_range(struct kvm *kvm, struct kvm_memory_slot *slot,
new_spte = mark_spte_for_access_track(new_spte);
}
- new_spte &= ~shadow_dirty_mask;
tdp_mmu_set_spte_no_acc_track(kvm, &iter, new_spte);
young = 1;
--
2.31.0.291.g576ba9dcdaf-goog
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH 2/2] KVM: x86/mmu: Simplify code for aging SPTEs in TDP MMU
2021-03-31 0:49 [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups Sean Christopherson
2021-03-31 0:49 ` [PATCH 1/2] KVM: x86/mmu: Remove spurious clearing of dirty bit from TDP MMU SPTE Sean Christopherson
@ 2021-03-31 0:49 ` Sean Christopherson
2021-03-31 9:36 ` [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups Paolo Bonzini
2 siblings, 0 replies; 4+ messages in thread
From: Sean Christopherson @ 2021-03-31 0:49 UTC (permalink / raw)
To: Paolo Bonzini
Cc: Sean Christopherson, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
Joerg Roedel, kvm, linux-kernel, Ben Gardon
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>
---
arch/x86/kvm/mmu/tdp_mmu.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 724088bea4b0..161b77925a19 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -951,7 +951,7 @@ 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();
@@ -966,8 +966,7 @@ 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
--
2.31.0.291.g576ba9dcdaf-goog
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups
2021-03-31 0:49 [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups Sean Christopherson
2021-03-31 0:49 ` [PATCH 1/2] KVM: x86/mmu: Remove spurious clearing of dirty bit from TDP MMU SPTE Sean Christopherson
2021-03-31 0:49 ` [PATCH 2/2] KVM: x86/mmu: Simplify code for aging SPTEs in TDP MMU Sean Christopherson
@ 2021-03-31 9:36 ` Paolo Bonzini
2 siblings, 0 replies; 4+ messages in thread
From: Paolo Bonzini @ 2021-03-31 9:36 UTC (permalink / raw)
To: Sean Christopherson
Cc: Vitaly Kuznetsov, Wanpeng Li, Jim Mattson, Joerg Roedel, kvm,
linux-kernel, Ben Gardon
On 31/03/21 02:49, Sean Christopherson wrote:
> Two minor fixes/cleanups for the TDP MMU, found by inspection.
>
> Sean Christopherson (2):
> KVM: x86/mmu: Remove spurious clearing of dirty bit from TDP MMU SPTE
> KVM: x86/mmu: Simplify code for aging SPTEs in TDP MMU
>
> arch/x86/kvm/mmu/tdp_mmu.c | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
Queued, thanks.
Paolo
^ permalink raw reply [flat|nested] 4+ messages in thread
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2021-03-31 0:49 [PATCH 0/2] KVM: x86/mmu: TDP MMU fixes/cleanups Sean Christopherson
2021-03-31 0:49 ` [PATCH 1/2] KVM: x86/mmu: Remove spurious clearing of dirty bit from TDP MMU SPTE Sean Christopherson
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