| Message ID | 20190507191805.9932-10-sean.j.christopherson@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: VMX: Reduce VMWRITEs to VMCS controls | expand |
On 07/05/19 21:18, Sean Christopherson wrote: > KVM dynamically toggles the CPU_BASED_USE_MSR_BITMAPS execution control > for nested guests based on whether or not both L0 and L1 want to pass > through the same MSRs to L2. Preserve the last used value from vmcs02 > so as to avoid multiple VMWRITEs to (re)set/(re)clear the bit on nested > VM-Entry. > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Needs a comment in code, and also leaves you puzzled as to why it's only necessary to clear it. We can even let the compiler merge the two "exec_control &= ~FOO" so that we don't even spend a precious instruction on the AND: diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index 1150831e4a9b..2e086d5ab837 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -2048,10 +2048,18 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) * A vmexit (to either L1 hypervisor or L0 userspace) is always needed * for I/O port accesses. */ - exec_control &= ~CPU_BASED_USE_IO_BITMAPS; exec_control |= CPU_BASED_UNCOND_IO_EXITING; - if (!(exec_controls_get(vmx) & CPU_BASED_USE_MSR_BITMAPS)) - exec_control &= ~CPU_BASED_USE_MSR_BITMAPS; + exec_control &= ~CPU_BASED_USE_IO_BITMAPS; + + /* + * This bit will be computed in nested_get_vmcs12_pages, because + * we do not have access to L1's MSR bitmap yet. For now, keep + * the same bit as before, hoping to avoid multiple VMWRITEs that + * only set/clear this bit. + */ + exec_control &= ~CPU_BASED_USE_MSR_BITMAPS; + exec_control |= exec_controls_get(vmx) & CPU_BASED_USE_MSR_BITMAPS; + exec_controls_set(vmx, exec_control); /* Paolo
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index cd1daf3b63a4..e4d363661ae7 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -2029,6 +2029,8 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12) */ exec_control &= ~CPU_BASED_USE_IO_BITMAPS; exec_control |= CPU_BASED_UNCOND_IO_EXITING; + if (!(exec_controls_get(vmx) & CPU_BASED_USE_MSR_BITMAPS)) + exec_control &= ~CPU_BASED_USE_MSR_BITMAPS; exec_controls_set(vmx, exec_control); /*
KVM dynamically toggles the CPU_BASED_USE_MSR_BITMAPS execution control for nested guests based on whether or not both L0 and L1 want to pass through the same MSRs to L2. Preserve the last used value from vmcs02 so as to avoid multiple VMWRITEs to (re)set/(re)clear the bit on nested VM-Entry. Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> --- arch/x86/kvm/vmx/nested.c | 2 ++ 1 file changed, 2 insertions(+)