linux-kernel - Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary

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Date:   Wed, 31 Mar 2021 21:22:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation
 unless necessary

On Wed, Mar 31, 2021, Sean Christopherson wrote:
> On Wed, Mar 31, 2021, Paolo Bonzini wrote:
> > On 31/03/21 21:47, Sean Christopherson wrote:
> > I also thought of busy waiting on down_read_trylock if the MMU notifier
> > cannot block, but that would also be invalid for the opposite reason (the
> > down_write task might be asleep, waiting for other readers to release the
> > task, and the down_read_trylock busy loop might not let that task run).
> > 
> > > And that's _already_ the worst case since notifications are currently
> > > serialized by mmu_lock.
> > 
> > But right now notifications are not a single critical section, they're two,
> > aren't they?
> 
> Ah, crud, yes.  Holding a spinlock across the entire start() ... end() would be
> bad, especially when the notifier can block since that opens up the possibility
> of the task sleeping/blocking/yielding while the spinlock is held.  Bummer.

On a related topic, any preference on whether to have an explicit "must_lock"
flag (what I posted), or derive the logic based on other params?

The helper I posted does:

	if (range->must_lock &&
	    kvm_mmu_lock_and_check_handler(kvm, range, &locked))
		goto out_unlock;

but it could be:

	if (!IS_KVM_NULL_FN(range->on_lock) && !range->may_block &&
	    kvm_mmu_lock_and_check_handler(kvm, range, &locked))
		goto out_unlock;

The generated code should be nearly identical on a modern compiler, so it's
purely a question of aesthetics.  I slightly prefer the explicit "must_lock" to
avoid spreading out the logic too much, but it also feels a bit superfluous.