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

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
c806e88734b9 ("x86/pkeys: Provide *pkru() helpers"), which renamed the raw
helper from __write_pkru() => wrpkru(), and turned __write_pkru() into a
wrapper.  Commit 577ff465f5a6 ("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: 577ff465f5a6 ("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>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7bbdf916a136913602b8d8ee22f1f421ea0ddfb7..46d12128f032c877f2a99aedf288e47eae28a4e3 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -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)) {