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KVM: x86: Load guest/host PKRU outside of the fastpath run loop

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Move KVM's swapping of PKRU outside of the fastpath loop, as there is no
KVM code anywhere in the fastpath that accesses guest/userspace memory,
i.e. that can consume protection keys.

As documented by commit 1be0e61c1f ("KVM, pkeys: save/restore PKRU when
guest/host switches"), KVM just needs to ensure the host's PKRU is loaded
when KVM (or the kernel at-large) may access userspace memory.  And at the
time of commit 1be0e61c1f, KVM didn't have a fastpath, and PKU was
strictly contained to VMX, i.e. there was no reason to swap PKRU outside
of vmx_vcpu_run().

Over time, the "need" to swap PKRU close to VM-Enter was likely falsely
solidified by the association with XFEATUREs in commit 37486135d3
("KVM: x86: Fix pkru save/restore when guest CR4.PKE=0, move it to x86.c"),
and XFEATURE swapping was in turn moved close to VM-Enter/VM-Exit as a
KVM hack-a-fix ution for an #MC handler bug by commit 1811d979c7
("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").

Deferring the PKRU loads shaves ~40 cycles off the fastpath for Intel,
and ~60 cycles for AMD.  E.g. using INVD in KVM-Unit-Test's vmexit.c,
with extra hacks to enable CR4.PKE and PKRU=(-1u & ~0x3), latency numbers
for AMD Turin go from ~1560 => ~1500, and for Intel Emerald Rapids, go
from ~810 => ~770.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Jon Kohler <jon@nutanix.com>
Link: https://patch.msgid.link/20251118222328.2265758-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2025-11-18 14:23:28 -08:00
parent 75c69c82f2
commit 7649412af3
4 changed files with 10 additions and 12 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
arch/x86/kvm/svm/svm.c
View File

@@ -4250,7 +4250,6 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
svm_set_dr6(vcpu, DR6_ACTIVE_LOW);
clgi();
kvm_load_guest_xsave_state(vcpu);
/*
* Hardware only context switches DEBUGCTL if LBR virtualization is
@@ -4293,7 +4292,6 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
vcpu->arch.host_debugctl != svm->vmcb->save.dbgctl)
update_debugctlmsr(vcpu->arch.host_debugctl);
kvm_load_host_xsave_state(vcpu);
stgi();
/* Any pending NMI will happen here */

4
arch/x86/kvm/vmx/vmx.c
View File

@@ -7473,8 +7473,6 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
vmx_set_interrupt_shadow(vcpu, 0);
kvm_load_guest_xsave_state(vcpu);
pt_guest_enter(vmx);
atomic_switch_perf_msrs(vmx);
@@ -7518,8 +7516,6 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
pt_guest_exit(vmx);
kvm_load_host_xsave_state(vcpu);
if (is_guest_mode(vcpu)) {
/*
* Track VMLAUNCH/VMRESUME that have made past guest state

14
arch/x86/kvm/x86.c
View File

@@ -1235,7 +1235,7 @@ static void kvm_load_host_xfeatures(struct kvm_vcpu *vcpu)
}
}
void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
static void kvm_load_guest_pkru(struct kvm_vcpu *vcpu)
{
if (vcpu->arch.guest_state_protected)
return;
@@ -1246,9 +1246,8 @@ void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE)))
wrpkru(vcpu->arch.pkru);
}
EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_load_guest_xsave_state);
void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
static void kvm_load_host_pkru(struct kvm_vcpu *vcpu)
{
if (vcpu->arch.guest_state_protected)
return;
@@ -1261,7 +1260,6 @@ void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
wrpkru(vcpu->arch.host_pkru);
}
}
EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_load_host_xsave_state);
#ifdef CONFIG_X86_64
static inline u64 kvm_guest_supported_xfd(struct kvm_vcpu *vcpu)
@@ -11303,6 +11301,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
guest_timing_enter_irqoff();
/*
* Swap PKRU with hardware breakpoints disabled to minimize the number
* of flows where non-KVM code can run with guest state loaded.
*/
kvm_load_guest_pkru(vcpu);
for (;;) {
/*
* Assert that vCPU vs. VM APICv state is consistent. An APICv
@@ -11331,6 +11335,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
++vcpu->stat.exits;
}
kvm_load_host_pkru(vcpu);
/*
* Do this here before restoring debug registers on the host. And
* since we do this before handling the vmexit, a DR access vmexit

2
arch/x86/kvm/x86.h
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@@ -636,8 +636,6 @@ static inline void kvm_machine_check(void)
#endif
}
void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu);
void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu);
int kvm_spec_ctrl_test_value(u64 value);
int kvm_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
struct x86_exception *e);