Re: [PATCH] KVM: nVMX: nested VPID emulation
From: Wanpeng Li
Date: Tue Sep 15 2015 - 06:15:34 EST
On 9/14/15 10:54 PM, Jan Kiszka wrote:
On 2015-09-14 14:52, Wanpeng Li wrote:
VPID is used to tag address space and avoid a TLB flush. Currently L0 use
the same VPID to run L1 and all its guests. KVM flushes VPID when switching
between L1 and L2.
This patch advertises VPID to the L1 hypervisor, then address space of L1 and
L2 can be separately treated and avoid TLB flush when swithing between L1 and
L2. This patch gets ~3x performance improvement for lmbench 8p/64k ctxsw.
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
arch/x86/kvm/vmx.c | 39 ++++++++++++++++++++++++++++++++-------
1 file changed, 32 insertions(+), 7 deletions(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index da1590e..06bc31e 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1157,6 +1157,11 @@ static inline bool nested_cpu_has_virt_x2apic_mode(struct vmcs12 *vmcs12)
return nested_cpu_has2(vmcs12, SECONDARY_EXEC_VIRTUALIZE_X2APIC_MODE);
}
+static inline bool nested_cpu_has_vpid(struct vmcs12 *vmcs12)
+{
+ return nested_cpu_has2(vmcs12, SECONDARY_EXEC_ENABLE_VPID);
+}
+
static inline bool nested_cpu_has_apic_reg_virt(struct vmcs12 *vmcs12)
{
return nested_cpu_has2(vmcs12, SECONDARY_EXEC_APIC_REGISTER_VIRT);
@@ -2471,6 +2476,7 @@ static void nested_vmx_setup_ctls_msrs(struct vcpu_vmx *vmx)
SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES |
SECONDARY_EXEC_RDTSCP |
SECONDARY_EXEC_VIRTUALIZE_X2APIC_MODE |
+ SECONDARY_EXEC_ENABLE_VPID |
SECONDARY_EXEC_APIC_REGISTER_VIRT |
SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY |
SECONDARY_EXEC_WBINVD_EXITING |
@@ -4160,7 +4166,7 @@ static void allocate_vpid(struct vcpu_vmx *vmx)
int vpid;
vmx->vpid = 0;
- if (!enable_vpid)
+ if (!enable_vpid || is_guest_mode(&vmx->vcpu))
return;
spin_lock(&vmx_vpid_lock);
vpid = find_first_zero_bit(vmx_vpid_bitmap, VMX_NR_VPIDS);
@@ -6738,6 +6744,14 @@ static int handle_vmclear(struct kvm_vcpu *vcpu)
}
vmcs12 = kmap(page);
vmcs12->launch_state = 0;
+ if (enable_vpid) {
+ if (nested_cpu_has_vpid(vmcs12)) {
+ spin_lock(&vmx_vpid_lock);
+ if (vmcs12->virtual_processor_id != 0)
+ __clear_bit(vmcs12->virtual_processor_id, vmx_vpid_bitmap);
+ spin_unlock(&vmx_vpid_lock);
Maybe enhance free_vpid (and also allocate_vpid) to work generically and
let the caller decide where to take the vpid from or where to store it?
Good idea.
+ }
+ }
kunmap(page);
nested_release_page(page);
@@ -9189,6 +9203,7 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
u32 exec_control;
+ int vpid;
vmcs_write16(GUEST_ES_SELECTOR, vmcs12->guest_es_selector);
vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector);
@@ -9438,13 +9453,21 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
else
vmcs_write64(TSC_OFFSET, vmx->nested.vmcs01_tsc_offset);
+
if (enable_vpid) {
- /*
- * Trivially support vpid by letting L2s share their parent
- * L1's vpid. TODO: move to a more elaborate solution, giving
- * each L2 its own vpid and exposing the vpid feature to L1.
- */
- vmcs_write16(VIRTUAL_PROCESSOR_ID, vmx->vpid);
+ if (nested_cpu_has_vpid(vmcs12)) {
+ if (vmcs12->virtual_processor_id == 0) {
+ spin_lock(&vmx_vpid_lock);
+ vpid = find_first_zero_bit(vmx_vpid_bitmap, VMX_NR_VPIDS);
+ if (vpid < VMX_NR_VPIDS)
+ __set_bit(vpid, vmx_vpid_bitmap);
+ spin_unlock(&vmx_vpid_lock);
+ vmcs_write16(VIRTUAL_PROCESSOR_ID, vpid);
It's a bit non-obvious that vpid == VMX_NR_VPIDS (no free vpids) will
lead to vpid == 0 when writing VIRTUAL_PROCESSOR_ID. You should leave at
least a comment. Or generalize allocate_vpid as that one is already
clearer in this regard.
Ditto.
+ } else
+ vmcs_write16(VIRTUAL_PROCESSOR_ID, vmcs12->virtual_processor_id);
+ } else
+ vmcs_write16(VIRTUAL_PROCESSOR_ID, vmx->vpid);
+
vmx_flush_tlb(vcpu);
}
@@ -9973,6 +9996,8 @@ static void prepare_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
vmcs12_save_pending_event(vcpu, vmcs12);
}
+ if (nested_cpu_has_vpid(vmcs12))
+ vmcs12->virtual_processor_id = vmcs_read16(VIRTUAL_PROCESSOR_ID);
/*
* Drop what we picked up for L2 via vmx_complete_interrupts. It is
* preserved above and would only end up incorrectly in L1.
Last but not least: the guest can now easily exhaust the host's pool of
vpid by simply spawning plenty of VCPUs for L2, no? Is this acceptable
or should there be some limit?
I reuse the value of vpid02 while vpid12 changed w/ one invvpid in v2,
and the scenario which you pointed out can be avoid.
Regards,
Wanpeng Li
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