Re: [PATCH v3 04/28] KVM: x86/mmu: Formalize TDP MMU's (unintended?) deferred TLB flush logic

[Mingwei Zhang]

On Sat, Feb 26, 2022, Sean Christopherson wrote:
> Explicitly ignore the result of zap_gfn_range() when putting the last
> reference to a TDP MMU root, and add a pile of comments to formalize the
> TDP MMU's behavior of deferring TLB flushes to alloc/reuse.  Note, this
> only affects the !shared case, as zap_gfn_range() subtly never returns
> true for "flush" as the flush is handled by tdp_mmu_zap_spte_atomic().
> 
> Putting the root without a flush is ok because even if there are stale
> references to the root in the TLB, they are unreachable because KVM will
> not run the guest with the same ASID without first flushing (where ASID
> in this context refers to both SVM's explicit ASID and Intel's implicit
> ASID that is constructed from VPID+PCID+EPT4A+etc...).
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c     |  8 ++++++++
>  arch/x86/kvm/mmu/tdp_mmu.c | 10 +++++++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 80607513a1f2..5a931c89d27b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5069,6 +5069,14 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
>  	kvm_mmu_sync_roots(vcpu);
>  
>  	kvm_mmu_load_pgd(vcpu);
> +
> +	/*
> +	 * Flush any TLB entries for the new root, the provenance of the root
> +	 * is unknown.  In theory, even if KVM ensures there are no stale TLB
> +	 * entries for a freed root, in theory, an out-of-tree hypervisor could
> +	 * have left stale entries.  Flushing on alloc also allows KVM to skip
> +	 * the TLB flush when freeing a root (see kvm_tdp_mmu_put_root()).
> +	 */
>  	static_call(kvm_x86_flush_tlb_current)(vcpu);
>  out:
>  	return r;
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 12866113fb4f..e35bd88d92fd 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -93,7 +93,15 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>  	list_del_rcu(&root->link);
>  	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
>  
> -	zap_gfn_range(kvm, root, 0, -1ull, false, false, shared);
> +	/*
> +	 * A TLB flush is not necessary as KVM performs a local TLB flush when
> +	 * allocating a new root (see kvm_mmu_load()), and when migrating vCPU
> +	 * to a different pCPU.  Note, the local TLB flush on reuse also
> +	 * invalidates any paging-structure-cache entries, i.e. TLB entries for
> +	 * intermediate paging structures, that may be zapped, as such entries
> +	 * are associated with the ASID on both VMX and SVM.
> +	 */
> +	(void)zap_gfn_range(kvm, root, 0, -1ull, false, false, shared);

Understood that we could avoid the TLB flush here. Just curious why the
"(void)" is needed here? Is it for compile time reason?
>  
>  	call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
>  }
> -- 
> 2.35.1.574.g5d30c73bfb-goog
>