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    SubjectRe: [PATCH 2/4] KVM: nVMX: track dirty state of non-shadowed VMCS fields
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    On 25/12/2017 04:03, Wanpeng Li wrote:
    > 2017-12-21 20:43 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
    >> VMCS12 fields that are not handled through shadow VMCS are rarely
    >> written, and thus they are also almost constant in the vmcs02. We can
    >> thus optimize prepare_vmcs02 by skipping all the work for non-shadowed
    >> fields in the common case.
    >>
    >> This patch introduces the (pretty simple) tracking infrastructure; the
    >> next patches will move work to prepare_vmcs02_full and save a few hundred
    >> clock cycles per VMRESUME on a Haswell Xeon E5 system:
    >>
    >> before after
    >> cpuid 14159 13869
    >> vmcall 15290 14951
    >> inl_from_kernel 17703 17447
    >> outl_to_kernel 16011 14692
    >> self_ipi_sti_nop 16763 15825
    >> self_ipi_tpr_sti_nop 17341 15935
    >> wr_tsc_adjust_msr 14510 14264
    >> rd_tsc_adjust_msr 15018 14311
    >> mmio-wildcard-eventfd:pci-mem 16381 14947
    >> mmio-datamatch-eventfd:pci-mem 18620 17858
    >> portio-wildcard-eventfd:pci-io 15121 14769
    >> portio-datamatch-eventfd:pci-io 15761 14831
    >>
    >> (average savings 748, stdev 460).
    >>
    >> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    >> ---
    >> arch/x86/kvm/vmx.c | 29 ++++++++++++++++++++++++++++-
    >> 1 file changed, 28 insertions(+), 1 deletion(-)
    >>
    >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
    >> index 2ee842990976..8b6013b529b3 100644
    >> --- a/arch/x86/kvm/vmx.c
    >> +++ b/arch/x86/kvm/vmx.c
    >> @@ -441,6 +441,7 @@ struct nested_vmx {
    >> * data hold by vmcs12
    >> */
    >> bool sync_shadow_vmcs;
    >> + bool dirty_vmcs12;
    >>
    >> bool change_vmcs01_virtual_x2apic_mode;
    >> /* L2 must run next, and mustn't decide to exit to L1. */
    >> @@ -7879,8 +7880,10 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu)
    >> {
    >> unsigned long field;
    >> gva_t gva;
    >> + struct vcpu_vmx *vmx = to_vmx(vcpu);
    >> unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
    >> u32 vmx_instruction_info = vmcs_read32(VMX_INSTRUCTION_INFO);
    >> +
    >> /* The value to write might be 32 or 64 bits, depending on L1's long
    >> * mode, and eventually we need to write that into a field of several
    >> * possible lengths. The code below first zero-extends the value to 64
    >> @@ -7923,6 +7926,20 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu)
    >> return kvm_skip_emulated_instruction(vcpu);
    >> }
    >>
    >> + switch (field) {
    >> +#define SHADOW_FIELD_RW(x) case x:
    >> +#include "vmx_shadow_fields.h"
    >
    > What's will happen here if enable_shadow_vmcs == false?

    If enable_shadow_vmcs == true, these fields never even go through
    handle_vmwrite, so they have to be written always by prepare_vmcs02.
    Other fields go through handle_vmwrite and set dirty_vmcs12.

    If enable_shadow_vmcs == false, or if the fields do not exist in the
    hardware VMCS (e.g. PML index on Haswell systems), writes go through
    handle_vmwrite, but they still don't need to go through
    prepare_vmcs02_full. So the switch statement recognizes these flags, so
    that they don't set dirty_vmcs12.

    Thanks,

    Paolo
    >
    > Regards,
    > Wanpeng Li
    >
    >> + /*
    >> + * The fields that can be updated by L1 without a vmexit are
    >> + * always updated in the vmcs02, the others go down the slow
    >> + * path of prepare_vmcs02.
    >> + */
    >> + break;
    >> + default:
    >> + vmx->nested.dirty_vmcs12 = true;
    >> + break;
    >> + }
    >> +
    >> nested_vmx_succeed(vcpu);
    >> return kvm_skip_emulated_instruction(vcpu);
    >> }
    >> @@ -7937,6 +7954,7 @@ static void set_current_vmptr(struct vcpu_vmx *vmx, gpa_t vmptr)
    >> __pa(vmx->vmcs01.shadow_vmcs));
    >> vmx->nested.sync_shadow_vmcs = true;
    >> }
    >> + vmx->nested.dirty_vmcs12 = true;
    >> }
    >>
    >> /* Emulate the VMPTRLD instruction */
    >> @@ -10569,6 +10587,11 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne
    >> return 0;
    >> }
    >>
    >> +static void prepare_vmcs02_full(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
    >> + bool from_vmentry)
    >> +{
    >> +}
    >> +
    >> /*
    >> * prepare_vmcs02 is called when the L1 guest hypervisor runs its nested
    >> * L2 guest. L1 has a vmcs for L2 (vmcs12), and this function "merges" it
    >> @@ -10864,7 +10887,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
    >> vmcs_write16(VIRTUAL_PROCESSOR_ID, vmx->vpid);
    >> vmx_flush_tlb(vcpu, true);
    >> }
    >> -
    >> }
    >>
    >> if (enable_pml) {
    >> @@ -10913,6 +10935,11 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
    >> /* Note: modifies VM_ENTRY/EXIT_CONTROLS and GUEST/HOST_IA32_EFER */
    >> vmx_set_efer(vcpu, vcpu->arch.efer);
    >>
    >> + if (vmx->nested.dirty_vmcs12) {
    >> + prepare_vmcs02_full(vcpu, vmcs12, from_vmentry);
    >> + vmx->nested.dirty_vmcs12 = false;
    >> + }
    >> +
    >> /* Shadow page tables on either EPT or shadow page tables. */
    >> if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),
    >> entry_failure_code))
    >> --
    >> 1.8.3.1
    >>
    >>
    >

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