linux-kernel - [PATCH] x86/fpu/xstate: clear XSAVE features if DISABLED

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Message-Id: <20230530200152.18961-1-jon@nutanix.com>
Date:   Tue, 30 May 2023 16:01:50 -0400
From:   Jon Kohler <jon@...anix.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Kyle Huey <me@...ehuey.com>, neelnatu@...gle.com,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Jon Kohler <jon@...anix.com>, linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH] x86/fpu/xstate: clear XSAVE features if DISABLED_MASK set

Respect DISABLED_MASK when clearing XSAVE features, such that features
that are disabled do not appear in the xfeatures mask.

This is important for kvm_load_{guest|host}_xsave_state, which look
at host_xcr0 and will do an expensive xsetbv when the guest and host
do not match.

A prime example if CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS is disabled,
the guest OS will not see PKU masked; however, the guest will incur
xsetbv since the host mask will never match the guest, even though
DISABLED_MASK16 has DISABLE_PKU set.

Signed-off-by: Jon Kohler <jon@...anix.com>
CC: kvm@...r.kernel.org
CC: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kernel/fpu/xstate.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 0bab497c9436..211ef82b53e3 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -798,7 +798,8 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 		unsigned short cid = xsave_cpuid_features[i];

 		/* Careful: X86_FEATURE_FPU is 0! */
-		if ((i != XFEATURE_FP && !cid) || !boot_cpu_has(cid))
+		if ((i != XFEATURE_FP && !cid) || !boot_cpu_has(cid) ||
+		    DISABLED_MASK_BIT_SET(cid))
 			fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
 	}

--
2.30.1 (Apple Git-130)