linux-kernel - Re: [PATCH 14/20] KVM: selftests: Collect *all* dirty entries in each dirty_log

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Message-ID: <93e571e9-5539-454f-9335-6de8339ffd8b@redhat.com>
Date: Thu, 19 Dec 2024 13:55:36 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 14/20] KVM: selftests: Collect *all* dirty entries in each
 dirty_log_test iteration

On 12/19/24 03:13, Sean Christopherson wrote:
> On Tue, Dec 17, 2024, Maxim Levitsky wrote:
>> While this patch might improve coverage for this particular case,
>> I think that this patch will make the test to be much more deterministic,
> 
> The verification will be more deterministic, but the actual testcase itself is
> just as random as it was before.

Based on my recollection of designing this thing with Peter, I can 
"confirm" that there was no particular intention of making the 
verification more random.

>> and thus have less chance of catching various races in the kernel that can happen.
>>
>> In fact in my option I prefer moving this test in other direction by
>> verifying dirty ring while the *vCPU runs* as well, in other words, not
>> stopping the vCPU at all unless its dirty ring is full.
> 
> But letting the vCPU-under-test keep changing the memory while it's being validated
> would add significant complexity, without any benefit insofar as I can see.  As
> evidenced by the bug the current approach can't detect, heavily stressing the
> system is meaningless if it's impossible to separate the signal from the noise.

Yes, I agree.

Paolo