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Date: Tue, 15 Sep 2015 18:14:57 +0800
From: Wanpeng Li <wanpeng.li@...mail.com>
To: Jan Kiszka <jan.kiszka@...mens.com>,
Paolo Bonzini <pbonzini@...hat.com>
CC: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: nested VPID emulation
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@...mail.com>
>> ---
>> 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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