Re: [RFC Patch 3/3] KVM/x86: Add tlb_remote_flush callback support for vmcs

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Tianyu Lan <Tianyu.Lan@microsoft.com> writes:

> Register tlb_remote_flush callback for vmcs when hyperv capability of
> nested guest mapping flush is detected. The interface can help to reduce
> overhead when flush ept table among vcpus for nested VM. The tradition way
> is to send IPIs to all affected vcpus and executes INVEPT on each vcpus.
> It will trigger several vmexits for IPI and INVEPT emulation. Hyperv provides
> such hypercall to do flush for all vcpus.
>
> Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
> ---
>  arch/x86/kvm/vmx.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index e50beb76d846..6cb241c05690 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4737,6 +4737,17 @@ static inline void __vmx_flush_tlb(struct kvm_vcpu *vcpu, int vpid,
>  	}
>  }
>
> +static int vmx_remote_flush_tlb(struct kvm *kvm)
> +{
> +	struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
> +
> +	if (!VALID_PAGE(vcpu->arch.mmu.root_hpa))
> +		return -1;

Why vcpu0? Can arch.mmu.root_hpa-s differ across vCPUs? What happens if
they do?

> +
> +	return hyperv_flush_guest_mapping(construct_eptp(vcpu,
> +		vcpu->arch.mmu.root_hpa));
> +}

The 'vmx_remote_flush_tlb' name looks generic enough but it is actually
Hyper-V-specific. I'd suggest renaming to something like
hv_remote_flush_tlb().

> +
>  static void vmx_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa)
>  {
>  	__vmx_flush_tlb(vcpu, to_vmx(vcpu)->vpid, invalidate_gpa);
> @@ -7495,6 +7506,10 @@ static __init int hardware_setup(void)
>  	if (enable_ept && !cpu_has_vmx_ept_2m_page())
>  		kvm_disable_largepages();
>
> +	if (ms_hyperv.nested_features & HV_X64_NESTED_GUSET_MAPPING_FLUSH
> +	    && enable_ept)
> +		kvm_x86_ops->tlb_remote_flush = vmx_remote_flush_tlb;
> +
>  	if (!cpu_has_vmx_ple()) {
>  		ple_gap = 0;
>  		ple_window = 0;

-- 
  Vitaly