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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aC0NMJIeqlgvq0yL@google.com>
Date: Tue, 20 May 2025 16:16:00 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Peter Xu <peterx@...hat.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, 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.

> The "serialization of concurrent RESETs" part could be a good side effect.
> After all, the dirty rings rely a lot on the userspace to do right things..
> for example, the userspace better also remember to reset before any slot
> changes, or it's possible to collect a dirty pfn with a slot index that was
> already removed and reused with a new one..
> 
> Maybe we could switch the sentences there in the comment of last patch, but
> not a huge deal.
> 
> Reviewed-by: Peter Xu <peterx@...hat.com>
> 
> Thanks!
> 
> -- 
> Peter Xu
>