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

From: Lai Jiangshan <laijs@linux.alibaba.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Avi Kivity <avi@redhat.com>,
	kvm@vger.kernel.org
Subject: Re: [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before link it
Date: Sat, 4 Sep 2021 00:25:27 +0800	[thread overview]
Message-ID: <7067bec0-8a15-1a18-481e-e2ea79575dcf@linux.alibaba.com> (raw)
In-Reply-To: <YTJIBr/lm5QU/Z3W@google.com>

On 2021/9/4 00:06, 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..2ff123ec0d64 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -704,6 +704,9 @@ 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 &&
> +			    mmu_sync_children(vcpu, sp, false))
> +				return RET_PF_RETRY;

It was like my first (unsent) fix.  Just return RET_PF_RETRY when break.

And then I thought that it'd be better to retry fetching directly rather than
retry guest when the conditions are still valid/unchanged to avoid all the
next guest page walking and GUP().  Although the code does not check all
conditions such as interrupt event pending. (we can add that too)

I think it is a good design to allow break mmu_lock when mmu is handling
heavy work.

>   		}
> 
>   		/*
> --
> 