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

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

On 2021/9/3 07:54, Sean Christopherson wrote:
> 
>   trace_get_page:
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 50ade6450ace..5b13918a55c2 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -704,6 +704,10 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>   			access = gw->pt_access[it.level - 2];
>   			sp = kvm_mmu_get_page(vcpu, table_gfn, fault->addr,
>   					      it.level-1, false, access);
> +			if (sp->unsync_children) {
> +				kvm_make_all_cpus_request(KVM_REQ_MMU_SYNC, vcpu);
> +				return RET_PF_RETRY;

Making KVM_REQ_MMU_SYNC be able remotely is good idea.
But if the sp is not linked, the @sp might not be synced even we
tried many times. So we should continue to link it.

But if we continue to link it, KVM_REQ_MMU_SYNC should be extended to
sync all roots (current root and prev_roots).  And maybe add a
KVM_REQ_MMU_SYNC_CURRENT for current root syncing.

It is not going to be a simple.  I have a new way to sync pages
and also fix the problem,  but that include several non-fix patches.

We need to fix this problem in the simplest way.  In my patch
mmu_sync_children() has a @root argument.  I think we can disallow
releasing the lock when @root is false. Is it OK?



> +			}
>   		}
> 
>   		/*
> --
>