linux-kernel - Re: [PATCH v2 0/7] Lazily allocate memslot rmaps

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Message-ID: <4d27e9d6-42db-3aa1-053a-552e1643f46d@redhat.com>
Date:   Tue, 4 May 2021 09:21:48 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...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 v2 0/7] Lazily allocate memslot rmaps

On 03/05/21 19:31, Ben Gardon wrote:
> On Mon, May 3, 2021 at 6:45 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> On 29/04/21 23:18, Ben Gardon wrote:
>>> This series enables KVM to save memory when using the TDP MMU by waiting
>>> to allocate memslot rmaps until they are needed. To do this, KVM tracks
>>> whether or not a shadow root has been allocated. In order to get away
>>> with not allocating the rmaps, KVM must also be sure to skip operations
>>> which iterate over the rmaps. If the TDP MMU is in use and we have not
>>> allocated a shadow root, these operations would essentially be op-ops
>>> anyway. Skipping the rmap operations has a secondary benefit of avoiding
>>> acquiring the MMU lock in write mode in many cases, substantially
>>> reducing MMU lock contention.
>>>
>>> This series was tested on an Intel Skylake machine. With the TDP MMU off
>>> and on, this introduced no new failures on kvm-unit-tests or KVM selftests.
>>
>> Thanks, I only reported some technicalities in the ordering of loads
>> (which matter since the loads happen with SRCU protection only).  Apart
>> from this, this looks fine!
> 
> Awesome to hear, thank you for the reviews. Should I send a v3
> addressing those comments, or did you already make those changes when
> applying to your tree?

No, I didn't (I wanted some oversight, and this is 5.14 stuff anyway).

Paolo