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KVM: VMX: Don't switch 64-bit msrs for 32-bit guests

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Some msrs are only used by x86_64 instructions, and are therefore
not needed when the guest is legacy mode.  By not bothering to switch
them, we reduce vmexit latency by 2400 cycles (from about 8800) when
running a 32-bt guest on a 64-bit host.

Signed-off-by: Avi Kivity <avi@qumranet.com>
This commit is contained in:
Avi Kivity 2007-04-19 13:22:48 +03:00
parent 2345df8c55
commit e38aea3e93
1 changed files with 42 additions and 16 deletions
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58
drivers/kvm/vmx.c
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@ -80,6 +80,9 @@ static const u32 vmx_msr_index[] = {
#ifdef CONFIG_X86_64
static unsigned msr_offset_kernel_gs_base;
#define NR_64BIT_MSRS 4
#else
#define NR_64BIT_MSRS 0
#endif
static inline int is_page_fault(u32 intr_info)
@ -300,6 +303,32 @@ static void vmx_inject_gp(struct kvm_vcpu *vcpu, unsigned error_code)
INTR_INFO_VALID_MASK);
}
/*
* Set up the vmcs to automatically save and restore system
* msrs. Don't touch the 64-bit msrs if the guest is in legacy
* mode, as fiddling with msrs is very expensive.
*/
static void setup_msrs(struct kvm_vcpu *vcpu)
{
int nr_skip, nr_good_msrs;
if (is_long_mode(vcpu))
nr_skip = NR_BAD_MSRS;
else
nr_skip = NR_64BIT_MSRS;
nr_good_msrs = vcpu->nmsrs - nr_skip;
vmcs_writel(VM_ENTRY_MSR_LOAD_ADDR,
virt_to_phys(vcpu->guest_msrs + nr_skip));
vmcs_writel(VM_EXIT_MSR_STORE_ADDR,
virt_to_phys(vcpu->guest_msrs + nr_skip));
vmcs_writel(VM_EXIT_MSR_LOAD_ADDR,
virt_to_phys(vcpu->host_msrs + nr_skip));
vmcs_write32(VM_EXIT_MSR_STORE_COUNT, nr_good_msrs); /* 22.2.2 */
vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, nr_good_msrs); /* 22.2.2 */
vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, nr_good_msrs); /* 22.2.2 */
}
/*
* reads and returns guest's timestamp counter "register"
* guest_tsc = host_tsc + tsc_offset -- 21.3
@ -825,6 +854,7 @@ static void vmx_set_efer(struct kvm_vcpu *vcpu, u64 efer)
msr->data = efer & ~EFER_LME;
}
setup_msrs(vcpu);
}
#endif
@ -988,7 +1018,6 @@ static int vmx_vcpu_setup(struct kvm_vcpu *vcpu)
struct descriptor_table dt;
int i;
int ret = 0;
int nr_good_msrs;
extern asmlinkage void kvm_vmx_return(void);
if (!init_rmode_tss(vcpu->kvm)) {
@ -1140,19 +1169,10 @@ static int vmx_vcpu_setup(struct kvm_vcpu *vcpu)
++vcpu->nmsrs;
}
nr_good_msrs = vcpu->nmsrs - NR_BAD_MSRS;
vmcs_writel(VM_ENTRY_MSR_LOAD_ADDR,
virt_to_phys(vcpu->guest_msrs + NR_BAD_MSRS));
vmcs_writel(VM_EXIT_MSR_STORE_ADDR,
virt_to_phys(vcpu->guest_msrs + NR_BAD_MSRS));
vmcs_writel(VM_EXIT_MSR_LOAD_ADDR,
virt_to_phys(vcpu->host_msrs + NR_BAD_MSRS));
setup_msrs(vcpu);
vmcs_write32_fixedbits(MSR_IA32_VMX_EXIT_CTLS, VM_EXIT_CONTROLS,
(HOST_IS_64 << 9)); /* 22.2,1, 20.7.1 */
vmcs_write32(VM_EXIT_MSR_STORE_COUNT, nr_good_msrs); /* 22.2.2 */
vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, nr_good_msrs); /* 22.2.2 */
vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, nr_good_msrs); /* 22.2.2 */
/* 22.2.1, 20.8.1 */
vmcs_write32_fixedbits(MSR_IA32_VMX_ENTRY_CTLS,
@ -1769,9 +1789,11 @@ again:
fx_restore(vcpu->guest_fx_image);
#ifdef CONFIG_X86_64
save_msrs(vcpu->host_msrs + msr_offset_kernel_gs_base, 1);
if (is_long_mode(vcpu)) {
save_msrs(vcpu->host_msrs + msr_offset_kernel_gs_base, 1);
load_msrs(vcpu->guest_msrs, NR_BAD_MSRS);
}
#endif
load_msrs(vcpu->guest_msrs, NR_BAD_MSRS);
asm (
/* Store host registers */
@ -1915,8 +1937,12 @@ again:
}
++kvm_stat.exits;
save_msrs(vcpu->guest_msrs, NR_BAD_MSRS);
load_msrs(vcpu->host_msrs, NR_BAD_MSRS);
#ifdef CONFIG_X86_64
if (is_long_mode(vcpu)) {
save_msrs(vcpu->guest_msrs, NR_BAD_MSRS);
load_msrs(vcpu->host_msrs, NR_BAD_MSRS);
}
#endif
fx_save(vcpu->guest_fx_image);
fx_restore(vcpu->host_fx_image);