linux-kernel - Re: [PATCH v4 2/5] arm64: KVM: use mutex_trylock_nest

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: <20250501134126.GT4439@noisy.programming.kicks-ass.net>
Date: Thu, 1 May 2025 15:41:26 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
	linux-riscv@...ts.infradead.org,
	Kunkun Jiang <jiangkunkun@...wei.com>,
	Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Boqun Feng <boqun.feng@...il.com>, Borislav Petkov <bp@...en8.de>,
	Albert Ou <aou@...s.berkeley.edu>, Anup Patel <anup@...infault.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Alexandre Ghiti <alex@...ti.fr>,
	Alexander Potapenko <glider@...gle.com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Andre Przywara <andre.przywara@....com>, x86@...nel.org,
	Joey Gouly <joey.gouly@....com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm-riscv@...ts.infradead.org,
	Atish Patra <atishp@...shpatra.org>, Ingo Molnar <mingo@...hat.com>,
	Jing Zhang <jingzhangos@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, kvmarm@...ts.linux.dev,
	Will Deacon <will@...nel.org>,
	Keisuke Nishimura <keisuke.nishimura@...ia.fr>,
	Sebastian Ott <sebott@...hat.com>, Shusen Li <lishusen2@...wei.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Zenghui Yu <yuzenghui@...wei.com>
Subject: Re: [PATCH v4 2/5] arm64: KVM: use mutex_trylock_nest_lock when
 locking all vCPUs

On Thu, May 01, 2025 at 01:44:28PM +0100, Marc Zyngier wrote:
> On Thu, 01 May 2025 12:15:52 +0100,
> Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > > + */
> > > > +int kvm_trylock_all_vcpus(struct kvm *kvm)
> > > > +{
> > > > +	struct kvm_vcpu *vcpu;
> > > > +	unsigned long i, j;
> > > > +
> > > > +	kvm_for_each_vcpu(i, vcpu, kvm)
> > > > +		if (!mutex_trylock_nest_lock(&vcpu->mutex, &kvm->lock))
> > 
> > This one includes an assertion that kvm->lock is actually held.
> 
> Ah, cunning. Thanks.
> 
> > That said, I'm not at all sure what the purpose of all this trylock
> > stuff is here.
> > 
> > Can someone explain? Last time I asked someone said something about
> > multiple VMs, but I don't know enough about kvm to know what that means.
> 
> Multiple VMs? That'd be real fun. Not.
> 
> > Are those vcpu->mutex another class for other VMs? Or what gives?
> 
> Nah. This is firmly single VM.
> 
> The purpose of this contraption is that there are some rare cases
> where we need to make sure that if we update some global state, all
> the vcpus of a VM need to see, or none of them.
> 
> For these cases, the guarantee comes from luserspace, and it gives the
> pinky promise that none of the vcpus are running at that point. But
> being of a suspicious nature, we assert that this is true by trying to
> take all the vcpu mutexes in one go. This will fail if a vcpu is
> running, as KVM itself takes the vcpu mutex before doing anything.
> 
> Similar requirement exists if we need to synthesise some state for
> userspace from all the individual vcpu states.

Ah, okay. Because x86 is simply doing mutex_lock() instead of
mutex_trylock() -- which would end up waiting for this activity to
subside I suppose.

Hence the use of the killable variant I suppose, for when they get tired
of waiting.

If all the architectures are basically doing the same thing, it might
make sense to unify this particular behaviour. But what do I know.