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

[Ben Gardon]

On Mon, May 3, 2021 at 6:45 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> 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?

>
> 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(-)
> >
>