linux-kernel - [PATCH] KVM: x86: use X86_FEATURE_RSB

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Message-Id: <20210507150636.94389-1-jon@nutanix.com>
Date:   Fri,  7 May 2021 11:06:36 -0400
From:   Jon Kohler <jon@...anix.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Jon Kohler <jon@...anix.com>, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: x86: use X86_FEATURE_RSB_CTXSW for RSB stuffing in vmexit

cpufeatures.h defines X86_FEATURE_RSB_CTXSW as "Fill RSB on context
switches" which seems more accurate than using X86_FEATURE_RETPOLINE
in the vmxexit path for RSB stuffing.

X86_FEATURE_RSB_CTXSW is used for FILL_RETURN_BUFFER in
arch/x86/entry/entry_{32|64}.S. This change makes KVM vmx and svm
follow that same pattern. This pairs up nicely with the language in
bugs.c, where this cpu_cap is enabled, which indicates that RSB
stuffing should be unconditional with spectrev2 enabled.
	/*
	 * If spectre v2 protection has been enabled, unconditionally fill
	 * RSB during a context switch; this protects against two independent
	 * issues:
	 *
	 *	- RSB underflow (and switch to BTB) on Skylake+
	 *	- SpectreRSB variant of spectre v2 on X86_BUG_SPECTRE_V2 CPUs
	 */
	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);

Furthermore, on X86_FEATURE_IBRS_ENHANCED CPUs && SPECTRE_V2_CMD_AUTO,
we're bypassing setting X86_FEATURE_RETPOLINE, where as far as I could
find, we should still be doing RSB stuffing no matter what when
CONFIG_RETPOLINE is enabled and spectrev2 is set to auto.

Signed-off-by: Jon Kohler <jon@...anix.com>
---
 arch/x86/kvm/svm/vmenter.S | 4 ++--
 arch/x86/kvm/vmx/vmenter.S | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index 4fa17df123cd..fe81012da4b5 100644
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -86,7 +86,7 @@ SYM_FUNC_START(__svm_vcpu_run)
 
 #ifdef CONFIG_RETPOLINE
 	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
-	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
+	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
 #endif
 
 	/* "POP" @regs to RAX. */
@@ -187,7 +187,7 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
 
 #ifdef CONFIG_RETPOLINE
 	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
-	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
+	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
 #endif
 
 	pop %_ASM_BX
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 3a6461694fc2..ede6aac7d8b7 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -76,12 +76,12 @@ SYM_FUNC_END(vmx_vmenter)
  */
 SYM_FUNC_START(vmx_vmexit)
 #ifdef CONFIG_RETPOLINE
-	ALTERNATIVE "jmp .Lvmexit_skip_rsb", "", X86_FEATURE_RETPOLINE
+	ALTERNATIVE "jmp .Lvmexit_skip_rsb", "", X86_FEATURE_RSB_CTXSW
 	/* Preserve guest's RAX, it's used to stuff the RSB. */
 	push %_ASM_AX
 
 	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
-	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
+	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
 
 	/* Clear RFLAGS.CF and RFLAGS.ZF to preserve VM-Exit, i.e. !VM-Fail. */
 	or $1, %_ASM_AX
-- 
2.30.1 (Apple Git-130)