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

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Message-ID: <c8cd9508-7516-0891-f507-4b869d7e4322@linux.alibaba.com>
Date:   Fri, 3 Sep 2021 08:44:53 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before
 link it

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?

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