Re: [PATCH v3] KVM: x86/MMU: Do not check unsync status for root SP.

[Yu Zhang]

Sorry, forget the change log:

Changes in V3:
- fixed a bug in warnings inside mmu_sync_children().
- commit message changes based on Paolo's suggestion.
- added Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Changes in V2:
- warnings added based on Sean's suggestion.


On Wed, Feb 10, 2021 at 01:01:11AM +0800, Yu Zhang wrote:
> In shadow page table, only leaf SPs may be marked as unsync;
> instead, for non-leaf SPs, we store the number of unsynced
> children in unsync_children. Therefore, in kvm_mmu_sync_root(),
> sp->unsync shall always be zero for the root SP and there is
> no need to check it. Remove the check, and add a warning
> inside mmu_sync_children() to assert that the flags are used
> properly.
> 
> While at it, move the warning from mmu_need_write_protect()
> to kvm_unsync_page().
> 
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 86af58294272..5f482af125b4 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1995,6 +1995,12 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
>  	LIST_HEAD(invalid_list);
>  	bool flush = false;
>  
> +	/*
> +	 * Only 4k SPTEs can directly be made unsync, the parent pages
> +	 * should never be unsyc'd.
> +	 */
> +	WARN_ON_ONCE(parent->unsync);
> +
>  	while (mmu_unsync_walk(parent, &pages)) {
>  		bool protected = false;
>  
> @@ -2502,6 +2508,8 @@ EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page);
>  
>  static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  {
> +	WARN_ON(sp->role.level != PG_LEVEL_4K);
> +
>  	trace_kvm_mmu_unsync_page(sp);
>  	++vcpu->kvm->stat.mmu_unsync;
>  	sp->unsync = 1;
> @@ -2524,7 +2532,6 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
>  		if (sp->unsync)
>  			continue;
>  
> -		WARN_ON(sp->role.level != PG_LEVEL_4K);
>  		kvm_unsync_page(vcpu, sp);
>  	}
>  
> @@ -3406,8 +3413,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
>  		 * mmu_need_write_protect() describe what could go wrong if this
>  		 * requirement isn't satisfied.
>  		 */
> -		if (!smp_load_acquire(&sp->unsync) &&
> -		    !smp_load_acquire(&sp->unsync_children))
> +		if (!smp_load_acquire(&sp->unsync_children))
>  			return;
>  
>  		write_lock(&vcpu->kvm->mmu_lock);
> -- 
> 2.17.1
>