Re: [PATCH v4 0/6] KVM: MMU: performance tweaks for heavy CR0.WP users

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On 23.03.23 23:50, Sean Christopherson wrote:
> On Wed, 22 Mar 2023 02:37:25 +0100, Mathias Krause wrote:
>> v3: https://lore.kernel.org/kvm/20230201194604.11135-1-minipli@xxxxxxxxxxxxxx/
>>
>> This series is the fourth iteration of resurrecting the missing pieces of
>> Paolo's previous attempt[1] to avoid needless MMU roots unloading.
>>
>> It's incorporating Sean's feedback to v3 and rebased on top of
>> kvm-x86/next, namely commit d8708b80fa0e ("KVM: Change return type of
>> kvm_arch_vm_ioctl() to "int"").
>>
>> [...]
> 
> Applied 1 and 5 to kvm-x86 mmu, and the rest to kvm-x86 misc, thanks!
> 
> [1/6] KVM: x86/mmu: Avoid indirect call for get_cr3
>       https://github.com/kvm-x86/linux/commit/2fdcc1b32418
> [2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled
>       https://github.com/kvm-x86/linux/commit/01b31714bd90
> [3/6] KVM: x86: Ignore CR0.WP toggles in non-paging mode
>       https://github.com/kvm-x86/linux/commit/e40bcf9f3a18
> [4/6] KVM: x86: Make use of kvm_read_cr*_bits() when testing bits
>       https://github.com/kvm-x86/linux/commit/74cdc836919b
> [5/6] KVM: x86/mmu: Fix comment typo
>       https://github.com/kvm-x86/linux/commit/50f13998451e
> [6/6] KVM: VMX: Make CR0.WP a guest owned bit
>       https://github.com/kvm-x86/linux/commit/fb509f76acc8

Thanks a lot, Sean!

As this is a huge performance fix for us, we'd like to get it integrated
into current stable kernels as well -- not without having the changes
get some wider testing, of course, i.e. not before they end up in a
non-rc version released by Linus. But I already did a backport to 5.4 to
get a feeling how hard it would be and for the impact it has on older
kernels.

Using the 'ssdd 10 50000' test I used before, I get promising results
there as well. Without the patches it takes 9.31s, while with them we're
down to 4.64s. Taking into account that this is the runtime of a
workload in a VM that gets cut in half, I hope this qualifies as stable
material, as it's a huge performance fix.

Greg, what's your opinion on it? Original series here:
https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@xxxxxxxxxxxxxx/

Thanks,
Mathias



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