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

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Date:   Tue, 4 May 2021 10:28:24 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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 Tue, May 4, 2021 at 12:21 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> 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).

Ah, okay I'll send out a v3 soon, discussion on the other patches settles.

>
> Paolo
>