Commit 4c20cd4c authored by Sean Christopherson's avatar Sean Christopherson
Browse files

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
c806e887 ("x86/pkeys: Provide *pkru() helpers"), which renamed the raw
helper from __write_pkru() => wrpkru(), and turned __write_pkru() into a
wrapper.  Commit 577ff465 ("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: default avatarAdrian Hunter <adrian.hunter@intel.com>
Fixes: 577ff465 ("x86/fpu: Only write PKRU if it is different from current")
Signed-off-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20241221011647.3747448-1-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent d6470627

arch/x86/kvm/x86.c

+2 −2
Original line number Diff line number Diff line
@@ -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)) {