linux-kernel - [PATCH v2 2/7] x86/kvm/vmx: Don't fetch the TSS base from the GDT

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Message-Id: <41f6d679c5620b5d4dd5746876f29085047bd5bf.1487704145.git.luto@kernel.org>
Date:   Tue, 21 Feb 2017 11:14:09 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>, X86 ML <x86@...nel.org>
Cc:     kvm list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Thomas Garnier <thgarnie@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: [PATCH v2 2/7] x86/kvm/vmx: Don't fetch the TSS base from the GDT

The current CPU's TSS base is a foregone conclusion, so there's no need
to parse it out of the segment tables.  This should save a couple cycles
(as STR is surely microcoded and poorly optimized) but, more importantly,
it's a cleanup and it means that segment_base() will never be called on
64-bit kernels.

Cc: Thomas Garnier <thgarnie@...gle.com>
Cc: Jim Mattson <jmattson@...gle.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kvm/vmx.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a236decb81e4..46420aaf1684 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2088,13 +2088,6 @@ static unsigned long segment_base(u16 selector)
 	return v;
 }
 
-static inline unsigned long kvm_read_tr_base(void)
-{
-	u16 tr;
-	asm("str %0" : "=g"(tr));
-	return segment_base(tr);
-}
-
 static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -2294,10 +2287,11 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
 		/*
 		 * Linux uses per-cpu TSS and GDT, so set these when switching
-		 * processors.
+		 * processors.  See 22.2.4.
 		 */
-		vmcs_writel(HOST_TR_BASE, kvm_read_tr_base()); /* 22.2.4 */
-		vmcs_writel(HOST_GDTR_BASE, gdt->address);   /* 22.2.4 */
+		vmcs_writel(HOST_TR_BASE,
+			    (unsigned long)this_cpu_ptr(&cpu_tss));
+		vmcs_writel(HOST_GDTR_BASE, gdt->address);
 
 		rdmsrl(MSR_IA32_SYSENTER_ESP, sysenter_esp);
 		vmcs_writel(HOST_IA32_SYSENTER_ESP, sysenter_esp); /* 22.2.3 */
-- 
2.9.3