linux-kernel - Re: [PATCH v3 0/6] KVM: Dirty ring fixes and cleanups

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Message-ID: <aC0VlENyfE9ewuTF@x1.local>
Date: Tue, 20 May 2025 19:51:48 -0400
From: Peter Xu <peterx@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yan Zhao <yan.y.zhao@...el.com>,
	Maxim Levitsky <mlevitsk@...hat.com>,
	Binbin Wu <binbin.wu@...ux.intel.com>,
	James Houghton <jthoughton@...gle.com>,
	Pankaj Gupta <pankaj.gupta@....com>
Subject: Re: [PATCH v3 0/6]  KVM: Dirty ring fixes and cleanups

On Tue, May 20, 2025 at 04:16:00PM -0700, Sean Christopherson wrote:
> On Tue, May 20, 2025, Peter Xu wrote:
> > On Fri, May 16, 2025 at 02:35:34PM -0700, Sean Christopherson wrote:
> > > Sean Christopherson (6):
> > >   KVM: Bound the number of dirty ring entries in a single reset at
> > >     INT_MAX
> > >   KVM: Bail from the dirty ring reset flow if a signal is pending
> > >   KVM: Conditionally reschedule when resetting the dirty ring
> > >   KVM: Check for empty mask of harvested dirty ring entries in caller
> > >   KVM: Use mask of harvested dirty ring entries to coalesce dirty ring
> > >     resets
> > >   KVM: Assert that slots_lock is held when resetting per-vCPU dirty
> > >     rings
> > 
> > For the last one, I'd think it's majorly because of the memslot accesses
> > (or CONFIG_LOCKDEP=y should yell already on resets?).  
> 
> No?  If KVM only needed to ensure stable memslot accesses, then SRCU would suffice.
> It sounds like holding slots_lock may have been a somewhat unintentional,  but the
> reason KVM can't switch to SRCU is that doing so would break ordering, not because
> slots_lock is needed to protect the memslot accesses.

Hmm.. isn't what you said exactly means a "yes"? :)

I mean, I would still expect lockdep to report this ioctl if without the
slots_lock, please correct me if it's not the case.  And if using RCU is
not trivial (or not necessary either), so far the slots_lock is still
required to make sure the memslot accesses are legal?

-- 
Peter Xu