linux-kernel - Re: [PATCH] KVM: x86/mmu: Don't rebuild page when the page is synced and no tlb flushing is required

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Message-ID: <c7309380-c7d5-419d-6ba5-25d3243c469b@redhat.com>
Date:   Fri, 25 Mar 2022 12:26:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Hou Wenlong <houwenlong.hwl@...group.com>
Cc:     kvm@...r.kernel.org, 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>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Don't rebuild page when the page is synced
 and no tlb flushing is required

On 3/24/22 21:17, Sean Christopherson wrote:
>> +static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>>   			 struct list_head *invalid_list)
>>   {
>>   	int ret = vcpu->arch.mmu->sync_page(vcpu, sp);
>>   
>> -	if (ret < 0) {
>> +	if (ret < 0)
>>   		kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list);
>> -		return false;
>> -	}
>> -
>> -	return !!ret;
>> +	return ret;
> Hrm, this creates an oddity in mmu_sync_children(), which does a logical-OR of
> the result into a boolean.  It doesn't actually change the functionality since
> kvm_mmu_remote_flush_or_zap() will prioritize invalid_list, but it's weird.
> 
> What about checking invalid_list directly and keeping the boolean return?  Compile
> tested only.

It's even better to check

         flush |= kvm_sync_page(vcpu, sp, &invalid_list) > 0;

in mmu_sync_children.  If the returned value is <0, then the page is 
added to invalid_list and there is no need to set flush = true, just 
like there is no need to call kvm_flush_remote_tlbs() in kvm_mmu_get_page().

Paolo