[PATCH][SVM] Only save/restore MSRs when needed

Anthony Liguori
2007-04-27T23:30:19Z

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I've tested this with 32-bit and 64-bit guests on a 64-bit host and with
32-bit guests on a 32-bit host.

I *think* it's doing the right thing wrt to DEBUGCTL but an extra set of
eyes would be helpful.

Depending on my machine's mood, we're at somewhere between 3200 and 2700
cycles per VMEXIT.

Regards,

Anthony Liguori

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From: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
Subject: [PATCH][SVM] Only save/restore MSRs when needed

We only have to save/restore MSR_GS_BASE on every VMEXIT. The rest can be
saved/restored when we leave the VCPU. As a special case, MSR_IA32_DEBUGCTL
only needs to be saved/restored on every exit if debugging is enabled in
either the host or the guest.

Signed-off-by: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>

Index: kernel/drivers/kvm/svm.c
===================================================================
--- kernel.orig/drivers/kvm/svm.c 2007-04-27 17:52:37.200780944 -0500
+++ kernel/drivers/kvm/svm.c 2007-04-27 18:14:02.098446808 -0500
@@ -611,7 +611,7 @@

static void svm_vcpu_load(struct kvm_vcpu *vcpu)
{
- int cpu;
+ int cpu, i;

cpu = get_cpu();
if (unlikely(cpu != vcpu->cpu)) {
@@ -626,10 +626,24 @@
vcpu->svm->vmcb->control.tsc_offset += delta;
vcpu->cpu = cpu;
}
+
+ for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
+ rdmsrl(host_save_user_msrs[i], vcpu->svm->host_user_msrs[i]);
+
+ rdmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
}

static void svm_vcpu_put(struct kvm_vcpu *vcpu)
{
+ int i;
+
+ for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
+ wrmsrl(host_save_user_msrs[i], vcpu->svm->host_user_msrs[i]);
+
+ wrmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
+ /* FIXME: LASTBRANCHFROMIP, LASTBRANCHTOIP, LASTINTFROMIP,
+ LASTINTTOIP */
+
rdtscll(vcpu->host_tsc);
put_cpu();
}
@@ -815,18 +829,16 @@

static void load_host_msrs(struct kvm_vcpu *vcpu)
{
- int i;
-
- for ( i = 0; i < NR_HOST_SAVE_MSRS; i++)
- wrmsrl(host_save_msrs[i], vcpu->svm->host_msrs[i]);
+#ifdef CONFIG_X86_64
+ wrmsrl(MSR_GS_BASE, vcpu->svm->host_gs_base);
+#endif
}

static void save_host_msrs(struct kvm_vcpu *vcpu)
{
- int i;
-
- for ( i = 0; i < NR_HOST_SAVE_MSRS; i++)
- rdmsrl(host_save_msrs[i], vcpu->svm->host_msrs[i]);
+#ifdef CONFIG_X86_64
+ rdmsrl(MSR_GS_BASE, vcpu->svm->host_gs_base);
+#endif
}

static void new_asid(struct kvm_vcpu *vcpu, struct svm_cpu_data *svm_data)
@@ -1498,6 +1510,11 @@
load_db_regs(vcpu->svm->db_regs);
}

+ if ((vcpu->svm->vmcb->save.dr7 & 0xff) ||
+ (vcpu->svm->host_dr7 & 0xff)) {
+ wrmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
+ }
+
if (vcpu->fpu_active) {
fx_save(vcpu->host_fx_image);
fx_restore(vcpu->guest_fx_image);
@@ -1617,6 +1634,11 @@
fx_restore(vcpu->host_fx_image);
}

+ if ((vcpu->svm->vmcb->save.dr7 & 0xff) ||
+ (vcpu->svm->host_dr7 & 0xff)) {
+ rdmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
+ }
+
if ((vcpu->svm->vmcb->save.dr7 & 0xff))
load_db_regs(vcpu->svm->host_db_regs);

Index: kernel/drivers/kvm/kvm_svm.h
===================================================================
--- kernel.orig/drivers/kvm/kvm_svm.h 2007-04-27 17:52:37.240774864 -0500
+++ kernel/drivers/kvm/kvm_svm.h 2007-04-27 17:53:42.509852456 -0500
@@ -9,17 +9,15 @@
#include "svm.h"
#include "kvm.h"

-static const u32 host_save_msrs[] = {
+static const u32 host_save_user_msrs[] = {
#ifdef CONFIG_X86_64
MSR_STAR, MSR_LSTAR, MSR_CSTAR, MSR_SYSCALL_MASK, MSR_KERNEL_GS_BASE,
- MSR_FS_BASE, MSR_GS_BASE,
+ MSR_FS_BASE,
#endif
MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP,
- MSR_IA32_DEBUGCTLMSR, /*MSR_IA32_LASTBRANCHFROMIP,
- MSR_IA32_LASTBRANCHTOIP, MSR_IA32_LASTINTFROMIP,MSR_IA32_LASTINTTOIP,*/
};

-#define NR_HOST_SAVE_MSRS ARRAY_SIZE(host_save_msrs)
+#define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs)
#define NUM_DB_REGS 4

struct vcpu_svm {
@@ -32,7 +30,9 @@

u64 next_rip;

- u64 host_msrs[NR_HOST_SAVE_MSRS];
+ u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS];
+ u64 host_ia32_debugctl;
+ u64 host_gs_base;
unsigned long host_cr2;
unsigned long host_db_regs[NUM_DB_REGS];
unsigned long host_dr6;

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