Re: [PATCH 06/15] KVM: nVMX: Don't "put" vCPU or host state when switching VMCS

Paolo Bonzini
2019-06-06T16:24:48Z

On 07/05/19 18:06, Sean Christopherson wrote:
> When switching between vmcs01 and vmcs02, KVM isn't actually switching
> between guest and host. If guest state is already loaded (the likely,
> if not guaranteed, case), keep the guest state loaded and manually swap
> the loaded_cpu_state pointer after propagating saved host state to the
> new vmcs0{1,2}.
>
> Avoiding the switch between guest and host reduces the latency of
> switching between vmcs01 and vmcs02 by several hundred cycles, and
> reduces the roundtrip time of a nested VM by upwards of 1000 cycles.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> arch/x86/kvm/vmx/nested.c | 18 +++++++++++++-
> arch/x86/kvm/vmx/vmx.c | 52 ++++++++++++++++++++++-----------------
> arch/x86/kvm/vmx/vmx.h | 3 ++-
> 3 files changed, 48 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index a30d53823b2e..4651d3462df4 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -241,15 +241,31 @@ static void free_nested(struct kvm_vcpu *vcpu)
> static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> + struct vmcs_host_state *src;
> + struct loaded_vmcs *prev;
> int cpu;
>
> if (vmx->loaded_vmcs == vmcs)
> return;
>
> cpu = get_cpu();
> - vmx_vcpu_put(vcpu);
> + prev = vmx->loaded_cpu_state;
> vmx->loaded_vmcs = vmcs;
> vmx_vcpu_load(vcpu, cpu);
> +
> + if (likely(prev)) {
> + src = &prev->host_state;
> +
> + vmx_set_host_fs_gs(&vmcs->host_state, src->fs_sel, src->gs_sel,
> + src->fs_base, src->gs_base);
> +
> + vmcs->host_state.ldt_sel = src->ldt_sel;
> +#ifdef CONFIG_X86_64
> + vmcs->host_state.ds_sel = src->ds_sel;
> + vmcs->host_state.es_sel = src->es_sel;
> +#endif
> + vmx->loaded_cpu_state = vmcs;
> + }
> put_cpu();

I'd like to extract this into a separate function:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 438fae1fef2a..83e436f201bf 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -248,34 +248,40 @@ static void free_nested(struct kvm_vcpu *vcpu)
free_loaded_vmcs(&vmx->nested.vmcs02);
}

+static void vmx_sync_vmcs_host_state(struct vcpu_vmx *vmx)
+{
+ struct loaded_vmcs *prev = vmx->loaded_cpu_state;
+ struct loaded_vmcs *cur;
+ struct vmcs_host_state *dest, *src;
+
+ if (unlikely(!prev))
+ return;
+
+ cur = &vmx->loaded_vmcs;
+ src = &prev->host_state;
+ dest = &cur->host_state;
+
+ vmx_set_host_fs_gs(dest, src->fs_sel, src->gs_sel, src->fs_base, src->gs_base);
+ dest->ldt_sel = src->ldt_sel;
+#ifdef CONFIG_X86_64
+ dest->ds_sel = src->ds_sel;
+ dest->es_sel = src->es_sel;
+#endif
+ vmx->loaded_cpu_state = cur;
+}
+
static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
- struct vmcs_host_state *src;
- struct loaded_vmcs *prev;
int cpu;

if (vmx->loaded_vmcs == vmcs)
return;

cpu = get_cpu();
- prev = vmx->loaded_cpu_state;
vmx->loaded_vmcs = vmcs;
vmx_vcpu_load(vcpu, cpu);
-
- if (likely(prev)) {
- src = &prev->host_state;
-
- vmx_set_host_fs_gs(&vmcs->host_state, src->fs_sel, src->gs_sel,
- src->fs_base, src->gs_base);
-
- vmcs->host_state.ldt_sel = src->ldt_sel;
-#ifdef CONFIG_X86_64
- vmcs->host_state.ds_sel = src->ds_sel;
- vmcs->host_state.es_sel = src->es_sel;
-#endif
- vmx->loaded_cpu_state = vmcs;
- }
+ vmx_sync_vmcs_host_state(vmx);
put_cpu();

vm_entry_controls_reset_shadow(vmx);

Paolo