[PATCH v3] KVM: x86: use wrpkru directly in kvm_load_{guest|host}_xsave_state

Jon Kohler
2021-05-11T17:07:00Z

kvm_load_host_xsave_state handles xsave on vm exit, part of which is
managing memory protection key state. The latest arch.pkru is updated
with a rdpkru, and if that doesn't match the base host_pkru (which
about 70% of the time), we issue a __write_pkru.

__write_pkru issues another rdpkru internally to try to avoid the
wrpkru, so we're reading the same value back to back when we 100% of
the time know that we need to go directly to wrpkru. This is a 100%
branch miss and extra work that can be skipped.

To improve performance, use wrpkru directly in KVM code and simplify
the uses of __write_pkru such that it can be removed completely.

While we're in this section of code, optimize if condition ordering
prior to wrpkru in both kvm_load_{guest|host}_xsave_state.

For both functions, flip the ordering of the || condition so that
arch.xcr0 & XFEATURE_MASK_PKRU is checked first, which when
instrumented in our evironment appeared to be always true and less
overall work than kvm_read_cr4_bits.

For kvm_load_guest_xsave_state, hoist arch.pkru != host_pkru ahead
one position. When instrumented, I saw this be true roughly ~70% of
the time vs the other conditions which were almost always true.
With this change, we will avoid 3rd condition check ~30% of the time.

Cc: Babu Moger <babu.moger@amd.com>
Signed-off-by: Jon Kohler <jon@nutanix.com>
---
v1 -> v2:
- Addressed comments on approach from Paolo and Dave.
v2 -> v3:
- fix return-too-soon in switch_fpu_finish, which would
have sometimes accidentally skipped update_pasid().
- make sure write_pkru() users do not regress by adding
pkru == rdpkru() optimization prior to wrpkru() call.

arch/x86/include/asm/fpu/internal.h | 8 +++++++-
arch/x86/include/asm/pgtable.h | 9 ++++++++-
arch/x86/include/asm/special_insns.h | 20 +++++---------------
arch/x86/kvm/x86.c | 14 +++++++-------
4 files changed, 27 insertions(+), 24 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 8d33ad80704f..5bc4df3a4c27 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -583,7 +583,13 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
if (pk)
pkru_val = pk->pkru;
}
- __write_pkru(pkru_val);
+
+ /*
+ * WRPKRU is relatively expensive compared to RDPKRU.
+ * Avoid WRPKRU when it would not change the value.
+ */
+ if (pkru_val != rdpkru())
+ wrpkru(pkru_val);

/*
* Expensive PASID MSR write will be avoided in update_pasid() because
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index b1099f2d9800..0bf9da90baaf 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -151,7 +151,14 @@ static inline void write_pkru(u32 pkru)
fpregs_lock();
if (pk)
pk->pkru = pkru;
- __write_pkru(pkru);
+
+ /*
+ * WRPKRU is relatively expensive compared to RDPKRU.
+ * Avoid WRPKRU when it would not change the value.
+ */
+ if (pkru != rdpkru())
+ wrpkru(pkru);
+
fpregs_unlock();
}

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 2acd6cb62328..3c361b5cbed5 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -99,32 +99,22 @@ static inline void wrpkru(u32 pkru)
/*
* "wrpkru" instruction. Loads contents in EAX to PKRU,
* requires that ecx = edx = 0.
+ * WRPKRU is relatively expensive compared to RDPKRU, callers
+ * should try to compare pkru == rdpkru() and avoid the call
+ * when it will not change the value; however, there are no
+ * correctness issues if a caller WRPKRU's for the same value.
*/
asm volatile(".byte 0x0f,0x01,0xef\n\t"
: : "a" (pkru), "c"(ecx), "d"(edx));
}

-static inline void __write_pkru(u32 pkru)
-{
- /*
- * WRPKRU is relatively expensive compared to RDPKRU.
- * Avoid WRPKRU when it would not change the value.
- */
- if (pkru == rdpkru())
- return;
-
- wrpkru(pkru);
-}
-
#else
static inline u32 rdpkru(void)
{
return 0;
}

-static inline void __write_pkru(u32 pkru)
-{
-}
+static inline void wrpkru(u32 pkru) {}
#endif

static inline void native_wbinvd(void)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cebdaa1e3cf5..3222c7f60f31 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -912,10 +912,10 @@ void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
}

if (static_cpu_has(X86_FEATURE_PKU) &&
- (kvm_read_cr4_bits(vcpu, X86_CR4_PKE) ||
- (vcpu->arch.xcr0 & XFEATURE_MASK_PKRU)) &&
- vcpu->arch.pkru != vcpu->arch.host_pkru)
- __write_pkru(vcpu->arch.pkru);
+ vcpu->arch.pkru != vcpu->arch.host_pkru &&
+ ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
+ kvm_read_cr4_bits(vcpu, X86_CR4_PKE)))
+ wrpkru(vcpu->arch.pkru);
}
EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);

@@ -925,11 +925,11 @@ void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
return;

if (static_cpu_has(X86_FEATURE_PKU) &&
- (kvm_read_cr4_bits(vcpu, X86_CR4_PKE) ||
- (vcpu->arch.xcr0 & XFEATURE_MASK_PKRU))) {
+ ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
+ kvm_read_cr4_bits(vcpu, X86_CR4_PKE))) {
vcpu->arch.pkru = rdpkru();
if (vcpu->arch.pkru != vcpu->arch.host_pkru)
- __write_pkru(vcpu->arch.host_pkru);
+ wrpkru(vcpu->arch.host_pkru);
}

if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE)) {
--
2.30.1 (Apple Git-130)