linux-kernel - Re: [PATCH v8 02/11] KVM: Add lockless memslot walk to KVM

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: <CADrL8HXEPAFmP8aONq6Df7HW_UPXYMAvAO+xYU6nwx1ZHLL0Ew@mail.gmail.com>
Date: Mon, 27 Jan 2025 11:51:31 -0800
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 02/11] KVM: Add lockless memslot walk to KVM

On Fri, Jan 10, 2025 at 2:26 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Nov 05, 2024, James Houghton wrote:
> > Provide flexibility to the architecture to synchronize as optimally as
>
> Similar to my nit on "locking" below, "synchronize" is somewhat misleading.  There's
> no requirement for architectures to synchronize during aging.  There is all but
> guaranteed to be some form of "synchronization", e.g. to prevent walking freed
> page tables, but the aging itself never synchronizes, and protecting a walk by
> disabling IRQs (as x86's shadow MMU does in some flows) only "synchronizes" in a
> loose sense of the word.
>
> > they can instead of always taking the MMU lock for writing.
> >
> > Architectures that do their own locking must select
> > CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS.
>
> This is backwards and could be misleading, and for the TDP MMU outright wrong.
> If some hypothetical architecture took _additional_ locks, then it can do so
> without needing to select CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS.
>
> What you want to say is that architectures that select
> CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS are responsible for ensuring correctness.
> And it's specifically all about correctness.  Common KVM doesn't care if the
> arch does it's own locking, e.g. taking mmu_lock for read, or has some completely
> lock-free scheme for aging.
>
> > The immediate application is to allow architectures to implement the
>
> "immediate application" is pretty redundant.  There's really only one reason to
> not take mmu_lock in this path.
>
> > test/clear_young MMU notifiers more cheaply.
>
> IMO, "more cheaply" is vague, and doesn't add much beyond the opening sentence
> about "synchronize as optimally as they can".  I would just delete this last
> sentence.

Thanks Sean. I've reworded the changelog like this:

    It is possible to correctly do aging without taking the KVM MMU lock;
    this option allows such architectures to do so. Architectures that
    select CONFIG_KVM_MMU_NOTIFIER_AGING_LOCKLESS are responsible for
    ensuring correctness.

>
> > Suggested-by: Yu Zhao <yuzhao@...gle.com>
> > Signed-off-by: James Houghton <jthoughton@...gle.com>
> > Reviewed-by: David Matlack <dmatlack@...gle.com>
> > ---
> > @@ -797,6 +805,8 @@ static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,
> >               .flush_on_ret   =
> >                       !IS_ENABLED(CONFIG_KVM_ELIDE_TLB_FLUSH_IF_YOUNG),
> >               .may_block      = false,
> > +             .lockless       =
> > +                     IS_ENABLED(CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS),
>
>                 .lockess        = IS_ENABLED(CONFIG_KVM_MMU_NOTIFIER_YOUNG_LOCKLESS),
>
> Random thought, maybe CONFIG_KVM_MMU_NOTIFIER_AGING_LOCKLESS or
> CONFIG_KVM_MMU_NOTIFIER_AGE_LOCKLESS instead of "YOUNG"?

CONFIG_KVM_MMU_NOTIFIER_AGING_LOCKLESS sounds good. Changed.