Re: [RFC 0/9] KVM:x86/mmu:Introduce parallel memory virtualization to boost performance

[Ben Gardon]

On Wed, Aug 5, 2020 at 9:53 AM Yulei Zhang <yulei.kernel@xxxxxxxxx> wrote:
>
> From: Yulei Zhang <yuleixzhang@xxxxxxxxxxx>
>
> Currently in KVM memory virtulization we relay on mmu_lock to synchronize
> the memory mapping update, which make vCPUs work in serialize mode and
> slow down the execution, especially after migration to do substantial
> memory mapping setup, and performance get worse if increase vCPU numbers
> and guest memories.
>
> The idea we present in this patch set is to mitigate the issue with
> pre-constructed memory mapping table. We will fast pin the guest memory
> to build up a global memory mapping table according to the guest memslots
> changes and apply it to cr3, so that after guest starts up all the vCPUs
> would be able to update the memory concurrently, thus the performance
> improvement is expected.

Is a re-implementation of the various MMU functions in this series
necessary to pre-populate the EPT/NPT? I realize the approach you took
is probably the fastest way to pre-populate an EPT, but it seems like
similar pre-population could be achieved with some changes to the PF
handler's prefault scheme or, from user space by adding a dummy vCPU
to touch memory before loading the actual guest image.

I think this series is taking a similar approach to the direct MMU RFC
I sent out a little less than a year ago. (I will send another version
of that series in the next month.) I'm not sure this level of
complexity is worth it if you're only interested in EPT pre-population.
Is pre-population your goal? You mention "parallel memory
virtualization," does that refer to parallel page fault handling you
intend to implement in a future series?

There are a number of features I see you've chosen to leave behind in
this series which might work for your use case, but I think they're
necessary. These include handling vCPUs with different roles (SMM, VMX
non root mode, etc.), MMU notifiers (which I realize matter less for
pinned memory), demand paging through UFFD, fast EPT
invalidation/teardown and others.

>
> And after test the initial patch with memory dirty pattern workload, we
> have seen positive results even with huge page enabled. For example,
> guest with 32 vCPUs and 64G memories, in 2M/1G huge page mode we would get
> more than 50% improvement.
>
>
> Yulei Zhang (9):
>   Introduce new fields in kvm_arch/vcpu_arch struct for direct build EPT
>     support
>   Introduce page table population function for direct build EPT feature
>   Introduce page table remove function for direct build EPT feature
>   Add release function for direct build ept when guest VM exit
>   Modify the page fault path to meet the direct build EPT requirement
>   Apply the direct build EPT according to the memory slots change
>   Add migration support when using direct build EPT
>   Introduce kvm module parameter global_tdp to turn on the direct build
>     EPT mode
>   Handle certain mmu exposed functions properly while turn on direct
>     build EPT mode
>
>  arch/mips/kvm/mips.c            |  13 +
>  arch/powerpc/kvm/powerpc.c      |  13 +
>  arch/s390/kvm/kvm-s390.c        |  13 +
>  arch/x86/include/asm/kvm_host.h |  13 +-
>  arch/x86/kvm/mmu/mmu.c          | 537 ++++++++++++++++++++++++++++++--
>  arch/x86/kvm/svm/svm.c          |   2 +-
>  arch/x86/kvm/vmx/vmx.c          |  17 +-
>  arch/x86/kvm/x86.c              |  55 ++--
>  include/linux/kvm_host.h        |   7 +-
>  virt/kvm/kvm_main.c             |  43 ++-
>  10 files changed, 648 insertions(+), 65 deletions(-)
>
> --
> 2.17.1
>