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

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Date:   Mon, 3 May 2021 15:44:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     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 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!

Paolo

> Changelog:
> v2:
> 	Incorporated feedback from Paolo and Sean
> 	Replaced the memslot_assignment_lock with slots_arch_lock, which
> 	has a larger critical section.
> 
> Ben Gardon (7):
>    KVM: x86/mmu: Track if shadow MMU active
>    KVM: x86/mmu: Skip rmap operations if shadow MMU inactive
>    KVM: x86/mmu: Deduplicate rmap freeing
>    KVM: x86/mmu: Factor out allocating memslot rmap
>    KVM: mmu: Refactor memslot copy
>    KVM: mmu: Add slots_arch_lock for memslot arch fields
>    KVM: x86/mmu: Lazily allocate memslot rmaps
> 
>   arch/x86/include/asm/kvm_host.h |  13 +++
>   arch/x86/kvm/mmu/mmu.c          | 153 +++++++++++++++++++++-----------
>   arch/x86/kvm/mmu/mmu_internal.h |   2 +
>   arch/x86/kvm/mmu/tdp_mmu.c      |   6 +-
>   arch/x86/kvm/mmu/tdp_mmu.h      |   4 +-
>   arch/x86/kvm/x86.c              | 110 +++++++++++++++++++----
>   include/linux/kvm_host.h        |   9 ++
>   virt/kvm/kvm_main.c             |  54 ++++++++---
>   8 files changed, 264 insertions(+), 87 deletions(-)
>