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

From: Paolo Bonzini
Date: Tue May 04 2021 - 03:21:56 EST

On 03/05/21 19:31, Ben Gardon wrote:
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?

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