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

From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
	Hou Wenlong <houwenlong.hwl@antgroup.com>
Cc: kvm@vger.kernel.org, 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>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Lai Jiangshan <laijs@linux.alibaba.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Don't rebuild page when the page is synced and no tlb flushing is required
Date: Fri, 25 Mar 2022 12:26:56 +0100	[thread overview]
Message-ID: <c7309380-c7d5-419d-6ba5-25d3243c469b@redhat.com> (raw)
In-Reply-To: <YjzRwDSPQNbpTrZ9@google.com>

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