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Subject: Re: [PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.
To:     Yu Zhang <yu.c.zhang@linux.intel.com>, seanjc@google.com
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com,
        joro@8bytes.org
References: <20210207122254.23056-1-yu.c.zhang@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <671ae214-22b9-1d89-75cb-0c6da5230988@redhat.com>
Date:   Mon, 8 Feb 2021 12:36:57 +0100
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On 07/02/21 13:22, Yu Zhang wrote:
> In shadow page table, only leaf SPs may be marked as unsync.
> And for non-leaf SPs, we use unsync_children to keep the number
> of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
> shall always be zero for the root SP, , hence no need to check
> it. Instead, a warning inside mmu_sync_children() is added, in
> case someone incorrectly used it.
> 
> Also, clarify the mmu_need_write_protect(), by moving the warning
> into kvm_unsync_page().
> 
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

This should really be more of a Co-developed-by, and there are a couple 
adjustments that could be made in the commit message.  I've queued the 
patch and I'll fix it up later.

Paolo

> ---
> Changes in V2:
> - warnings added based on Sean's suggestion.
> 
>   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 86af582..c4797a00cc 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(sp->unsync);
> +
>   	while (mmu_unsync_walk(parent, &pages)) {
>   		bool protected = false;
>   
> @@ -2502,6 +2508,8 @@ int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
>   
>   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);
>