linux-kernel - [PATCH] KVM: x86: Avoid double RDPKRU when loading host/guest PKRU

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Message-ID: <20241221011647.3747448-1-seanjc@google.com>
Date: Fri, 20 Dec 2024 17:16:47 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Adrian Hunter <adrian.hunter@...el.com>
Subject: [PATCH] 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@...el.com>
Fixes: 577ff465f5a6 ("x86/fpu: Only write PKRU if it is different from current")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4320647bd78a..9d5cece9260b 100644
--- 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)) {

base-commit: 13e98294d7cec978e31138d16824f50556a62d17
-- 
2.47.1.613.gc27f4b7a9f-goog