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

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Message-ID: <c3bd7db9-2356-2bb3-4869-7c3edf922e53@linux.alibaba.com>
Date:   Sat, 4 Sep 2021 01:00:33 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        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 2021/9/4 00:40, Sean Christopherson wrote:
> On Sat, Sep 04, 2021, Lai Jiangshan wrote:
>>
>> On 2021/9/4 00:06, Sean Christopherson wrote:
>>
>>> 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)
> 
> But not in a bug fix that needs to go to stable branches.

Good point, it is too complicated for a fix, I accept just "return RET_PF_RETRY".
(and don't need "SOME_ARBITRARY_THRESHOLD").

Is it Ok? I will update the patch as it.

>   
>> I think it is a good design to allow break mmu_lock when mmu is handling
>> heavy work.
> 
> I don't disagree in principle, but I question the relevance/need.  I doubt this
> code is relevant to nested TDP performance as hypervisors generally don't do the
> type of PTE manipulations that would lead to linking an existing unsync sp.  And
> for legacy shadow paging, my preference would be to put it into maintenance-only
> mode as much as possible.  I'm not dead set against new features/functionality
> for shadow paging, but for something like dropping mmu_lock in the page fault path,
> IMO there needs to be performance numbers to justify such a change.
> 

I understood the concern and the relevance/need.