Re: [PATCH v4 17/30] KVM: x86/mmu: Require mmu_lock be held for write to zap TDP MMU range

[Mingwei Zhang <mizhang@google.com>]

On Thu, Mar 03, 2022, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> Now that all callers of zap_gfn_range() hold mmu_lock for write, drop
> support for zapping with mmu_lock held for read.  That all callers hold
> mmu_lock for write isn't a random coincidence; now that the paths that
> need to zap _everything_ have their own path, the only callers left are
> those that need to zap for functional correctness.  And when zapping is
> required for functional correctness, mmu_lock must be held for write,
> otherwise the caller has no guarantees about the state of the TDP MMU
> page tables after it has run, e.g. the SPTE(s) it zapped can be
> immediately replaced by a vCPU faulting in a page.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Ben Gardon <bgardon@google.com>
> Message-Id: <20220226001546.360188-17-seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 24 ++++++------------------
>  1 file changed, 6 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 970376297b30..f3939ce4a115 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -844,15 +844,9 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
>   * function cannot yield, it will not release the MMU lock or reschedule and
>   * the caller must ensure it does not supply too large a GFN range, or the
>   * operation can cause a soft lockup.
> - *
> - * If shared is true, this thread holds the MMU lock in read mode and must
> - * account for the possibility that other threads are modifying the paging
> - * structures concurrently. If shared is false, this thread should hold the
> - * MMU lock in write mode.
>   */
>  static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> -			  gfn_t start, gfn_t end, bool can_yield, bool flush,
> -			  bool shared)
> +			  gfn_t start, gfn_t end, bool can_yield, bool flush)
>  {
>  	bool zap_all = (start == 0 && end >= tdp_mmu_max_gfn_host());
>  	struct tdp_iter iter;
> @@ -865,14 +859,13 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>  
>  	end = min(end, tdp_mmu_max_gfn_host());
>  
> -	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
> +	lockdep_assert_held_write(&kvm->mmu_lock);
>  
>  	rcu_read_lock();
>  
>  	for_each_tdp_pte_min_level(iter, root, min_level, start, end) {
> -retry:
>  		if (can_yield &&
> -		    tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) {
> +		    tdp_mmu_iter_cond_resched(kvm, &iter, flush, false)) {
>  			flush = false;
>  			continue;
>  		}
> @@ -891,12 +884,8 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>  		    !is_last_spte(iter.old_spte, iter.level))
>  			continue;
>  
> -		if (!shared) {
> -			tdp_mmu_set_spte(kvm, &iter, 0);
> -			flush = true;
> -		} else if (tdp_mmu_zap_spte_atomic(kvm, &iter)) {
> -			goto retry;
> -		}
> +		tdp_mmu_set_spte(kvm, &iter, 0);
> +		flush = true;
>  	}
>  
>  	rcu_read_unlock();
> @@ -915,8 +904,7 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
>  	struct kvm_mmu_page *root;
>  
>  	for_each_tdp_mmu_root_yield_safe(kvm, root, as_id)
> -		flush = zap_gfn_range(kvm, root, start, end, can_yield, flush,
> -				      false);
> +		flush = zap_gfn_range(kvm, root, start, end, can_yield, flush);
>  
>  	return flush;
>  }
> -- 
> 2.31.1
> 
>