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

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Date:   Tue, 17 Jan 2023 15:42:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Gavin Shan <gshan@...hat.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()

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")