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KVM: x86: Avoid double RDPKRU when loading host/guest PKRU

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Use the raw wrpkru() helper when loading the guest/host's PKRU on switch
to/from guest context, as the write_pkru() wrapper incurs an unnecessary
rdpkru().  In both paths, KVM is guaranteed to have performed RDPKRU since
the last possible write, i.e. KVM has a fresh cache of the current value
in hardware.

This effectively restores KVM's behavior to that of KVM prior to commit
c806e88734 ("x86/pkeys: Provide *pkru() helpers"), which renamed the raw
helper from __write_pkru() => wrpkru(), and turned __write_pkru() into a
wrapper.  Commit 577ff465f5 ("x86/fpu: Only write PKRU if it is different
from current") then added the extra RDPKRU to avoid an unnecessary WRPKRU,
but completely missed that KVM already optimized away pointless writes.

Reported-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 577ff465f5 ("x86/fpu: Only write PKRU if it is different from current")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20241221011647.3747448-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2024-12-20 17:16:47 -08:00
parent d6470627f5
commit 4c20cd4cee
1 changed files with 2 additions and 2 deletions
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4
arch/x86/kvm/x86.c
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@@ -1186,7 +1186,7 @@ void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
vcpu->arch.pkru != vcpu->arch.host_pkru &&
((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE)))
write_pkru(vcpu->arch.pkru);
wrpkru(vcpu->arch.pkru);
}
EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);
@@ -1200,7 +1200,7 @@ void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
kvm_is_cr4_bit_set(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_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE)) {