linux-kernel - Re: [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond

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Date:   Tue, 26 Jan 2021 15:38:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond_resched and needbreak

On 21/01/21 01:19, Sean Christopherson wrote:
> What if we simply make the common mmu_lock a union? The rwlock_t is 
> probably a bit bigger, but that's a few bytes for an entire VM. And 
> maybe this would entice/inspire other architectures to move to a similar 
> MMU model.

Looking more at this, there is a problem in that MMU notifier functions 
take the MMU lock.

Yes, qrwlock the size is a bit larger than qspinlock.  However, the fast 
path of qrwlocks is small, and if the slow paths are tiny compared to 
the mmu_lock critical sections that are so big as to require 
cond_resched.  So I would consider just changing all architectures to an 
rwlock.

Paolo