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

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Message-ID: <743a31e7-03ba-0c64-86ac-c5a0aac4121c@redhat.com>
Date:   Wed, 31 Mar 2021 23:36:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 31/03/21 23:22, Sean Christopherson wrote:
> 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.

I do as well, but I hope we don't need any lock after all as in the 
email I've just sent.

Paolo