linux-kernel - Re: [PATCH 4/4] KVM: Improve warning report in mark_page_dirty_in

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Date:   Fri, 20 Jan 2023 10:06:52 +1100
From:   Gavin Shan <gshan@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvmarm@...ts.linux.dev, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, maz@...nel.org, corbet@....net,
        james.morse@....com, suzuki.poulose@....com,
        oliver.upton@...ux.dev, yuzenghui@...wei.com,
        catalin.marinas@....com, will@...nel.org, ricarkol@...gle.com,
        eric.auger@...hat.com, yuzhe@...china.com, renzhengeek@...il.com,
        ardb@...nel.org, peterx@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH 4/4] KVM: Improve warning report in
 mark_page_dirty_in_slot()

Hi Sean,

On 1/20/23 2:19 AM, Sean Christopherson wrote:
> On Thu, Jan 19, 2023, Gavin Shan wrote:
>> On 1/18/23 2:42 AM, Sean Christopherson wrote:
>>> On Mon, Jan 16, 2023, Gavin Shan wrote:
>>>> There are two warning reports about the dirty ring in the function.
>>>> We have the wrong assumption that the dirty ring is always enabled when
>>>> CONFIG_HAVE_KVM_DIRTY_RING is selected.
>>>
>>> No, it's not a wrong assumption, becuase it's not an assumption.  The intent is
>>> to warn irrespective of dirty ring/log enabling.  The orignal code actually warned
>>> irrespective of dirty ring support[1], again intentionally.  The
>>> CONFIG_HAVE_KVM_DIRTY_RING check was added because s390 can mark pages dirty from
>>> an worker thread[2] and s390 has no plans to support the dirty ring.
>>>
>>> The reason for warning even if dirty ring isn't enabled is so that bots can catch
>>> potential KVM bugs without having to set up a dirty ring or enable dirty logging.
>>>
>>> [1] 2efd61a608b0 ("KVM: Warn if mark_page_dirty() is called without an active vCPU")
>>> [2] e09fccb5435d ("KVM: avoid warning on s390 in mark_page_dirty")
>>>
>>
>> Thanks for the linker. I was confused when looking at the code, but now it's clear to
>> me. Thanks for your explanation. How about to add a comment there?
>>
>>    /*
>>     * The warning is expected when the dirty ring is configured,
>>     * but not enabled.
>>     */
> 
> That's not correct either.  By design, the warning can also fire if the dirty ring
> is enabled.  KVM's rule is that writes to guest memory always need to be done in
> the context of a running vCPU, with the recently added exception of
> kvm_arch_allow_write_without_running_vcpu().  That intent of the warning is to
> enforce that rule regardless of the state of the VM.
> 
> Concretely, I think you can just drop patches 3 and 4, and just fix the arm64 issues.
> 

Right, the warning report is still expected when dirty ring is enabled. My attempt
was to have comment for the confused case. Anyway, it's not a big deal. I will drop
PATCH[3] and PATCH[4] in v2.

Thanks,
Gavin