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
c806e88 ("x86/pkeys: Provide *pkru() helpers"), which renamed the raw
helper from __write_pkru() => wrpkru(), and turned __write_pkru() into a
wrapper.  Commit 577ff46 ("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: 577ff46 ("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>

Author: sean-jc

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)) {