linux-kernel - [PATCH 1/4] kvm, vmx: move CR2 context switch out of assembly path

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Date:   Wed, 24 Oct 2018 10:28:56 +0200
From:   Julian Stecklina <jsteckli@...zon.de>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Julian Stecklina <jsteckli@...zon.de>, js@...en8.de,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/4] kvm, vmx: move CR2 context switch out of assembly path

The VM entry/exit path is a giant inline assembly statement. Simplify it
by doing CR2 context switching in plain C. Move CR2 restore behind IBRS
clearing, so we reduce the amount of code we execute with IBRS on.

Signed-off-by: Julian Stecklina <jsteckli@...zon.de>
Reviewed-by: Jan H. Schönherr <jschoenh@...zon.de>
Reviewed-by: Konrad Jan Miller <kjm@...zon.de>
---
 arch/x86/kvm/vmx.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index e665aa7..93562d5 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10728,6 +10728,9 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	evmcs_rsp = static_branch_unlikely(&enable_evmcs) ?
 		(unsigned long)&current_evmcs->host_rsp : 0;
 
+	if (read_cr2() != vcpu->arch.cr2)
+		write_cr2(vcpu->arch.cr2);
+
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
 
@@ -10747,13 +10750,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 		"2: \n\t"
 		__ex(ASM_VMX_VMWRITE_RSP_RDX) "\n\t"
 		"1: \n\t"
-		/* Reload cr2 if changed */
-		"mov %c[cr2](%0), %%" _ASM_AX " \n\t"
-		"mov %%cr2, %%" _ASM_DX " \n\t"
-		"cmp %%" _ASM_AX ", %%" _ASM_DX " \n\t"
-		"je 3f \n\t"
-		"mov %%" _ASM_AX", %%cr2 \n\t"
-		"3: \n\t"
 		/* Check if vmlaunch of vmresume is needed */
 		"cmpl $0, %c[launched](%0) \n\t"
 		/* Load guest registers.  Don't clobber flags. */
@@ -10810,8 +10806,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 		"xor %%r14d, %%r14d \n\t"
 		"xor %%r15d, %%r15d \n\t"
 #endif
-		"mov %%cr2, %%" _ASM_AX "   \n\t"
-		"mov %%" _ASM_AX ", %c[cr2](%0) \n\t"
 
 		"xor %%eax, %%eax \n\t"
 		"xor %%ebx, %%ebx \n\t"
@@ -10843,7 +10837,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 		[r14]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_R14])),
 		[r15]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_R15])),
 #endif
-		[cr2]"i"(offsetof(struct vcpu_vmx, vcpu.arch.cr2)),
 		[wordsize]"i"(sizeof(ulong))
 	      : "cc", "memory"
 #ifdef CONFIG_X86_64
@@ -10877,6 +10870,8 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	/* Eliminate branch target predictions from guest mode */
 	vmexit_fill_RSB();
 
+	vcpu->arch.cr2 = read_cr2();
+
 	/* All fields are clean at this point */
 	if (static_branch_unlikely(&enable_evmcs))
 		current_evmcs->hv_clean_fields |=
-- 
2.7.4