Re: [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before link it

[Lai Jiangshan <laijs@linux.alibaba.com>]

On 2021/9/4 00:06, Sean Christopherson wrote:

> -static void mmu_sync_children(struct kvm_vcpu *vcpu,
> -			      struct kvm_mmu_page *parent)
> +static int mmu_sync_children(struct kvm_vcpu *vcpu,
> +			     struct kvm_mmu_page *parent, bool can_yield)
>   {
>   	int i;
>   	struct kvm_mmu_page *sp;
> @@ -2050,7 +2050,15 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
>   			flush |= kvm_sync_page(vcpu, sp, &invalid_list);
>   			mmu_pages_clear_parents(&parents);
>   		}
> -		if (need_resched() || rwlock_needbreak(&vcpu->kvm->mmu_lock)) {
> +		/*
> +		 * Don't yield if there are fewer than <N> unsync children
> +		 * remaining, just finish up and get out.
> +		 */
> +		if (parent->unsync_children > SOME_ARBITRARY_THRESHOLD &&
> +		    (need_resched() || rwlock_needbreak(&vcpu->kvm->mmu_lock))) {
> +			if (!can_yield)
> +				return -EINTR;
> +


Another thought about this function.

It is courtesy to break when rwlock_needbreak(), but the price is quite high,
with remote flushing to interrupt several pCPUs.  I think we can only break
when need_resched().