Re: [PATCH] KVM: x86: Optimization: Requst TLB flush in fast_cr3_switch() instead of do it directly

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Nov 12, 2019 at 08:33:00PM +0200, Liran Alon wrote:
> When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
> page. If nEPT is used, this will happen from
> kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
> happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
> __kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
> previously cached root page.
> 
> In case fast_cr3_switch() finds a matching cached root page, it will
> set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
> next entry to guest, KVM will set root HPA in appropriate hardware
> fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
> kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
> was replaced.
> 
> This works as mmu->root_hpa, which vmx_flush_tlb() use, was
> already replaced in cached_root_available(). However, this may
> result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
> may have already been requested. For example, by prepare_vmcs02()
> in case L1 don't use VPID.
> 
> Therefore, change fast_cr3_switch() to just request TLB flush on
> next entry to guest.
> 
> Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>  arch/x86/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 24c23c66b226..150d982ec1d2 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4295,7 +4295,7 @@ static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3,
>  			kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);
>  			if (!skip_tlb_flush) {
>  				kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> -				kvm_x86_ops->tlb_flush(vcpu, true);
> +				kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);

Ha, I brought this up in the original review[*].  Junaid was concerned with
maintaining the existing behavior for vcpu->stat.tlb_flush, so we kept the
direct call to ->tlb_flush().  Incrementing tlb_flush seems a-ok, so:

Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

[*] https://patchwork.kernel.org/patch/10461319/#21985483

>  			}
>  
>  			/*
> -- 
> 2.20.1
>