linux-kernel - Re: [PATCH] KVM: x86/mmu: Add "never" option to allow sticky disabling of nx_huge

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Message-ID: <4b621470-8c58-264b-1e8b-75cec73cd7b0@gmail.com>
Date:   Wed, 12 Jul 2023 00:12:49 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Luiz Capitulino <luizcap@...zon.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Li RongQing <lirongqing@...du.com>,
        Yong He <zhuangel570@...il.com>,
        Robert Hoo <robert.hoo.linux@...il.com>,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH] KVM: x86/mmu: Add "never" option to allow sticky
 disabling of nx_huge_pages

On 2023/6/15 03:07, Sean Christopherson wrote:
> On Wed, Jun 14, 2023, Luiz Capitulino wrote:
>>
>>
>> On 2023-06-13 19:21, Sean Christopherson wrote:
>>
>>>
>>>
>>>
>>> On Thu, 01 Jun 2023 17:58:59 -0700, Sean Christopherson wrote:
>>>> Add a "never" option to the nx_huge_pages module param to allow userspace
>>>> to do a one-way hard disabling of the mitigation, and don't create the
>>>> per-VM recovery threads when the mitigation is hard disabled.  Letting
>>>> userspace pinky swear that userspace doesn't want to enable NX mitigation
>>>> (without reloading KVM) allows certain use cases to avoid the latency
>>>> problems associated with spawning a kthread for each VM.
>>>>
>>>> [...]
>>>
>>> Applied to kvm-x86 mmu.  I kept the default as "auto" for now, as that can go on
>>> top and I don't want to introduce that change this late in the cycle.  If no one
>>> beats me to the punch (hint, hint ;-) ), I'll post a patch to make "never" the
>>> default for unaffected hosts so that we can discuss/consider that change for 6.6.
>>
>> Thanks Sean, I agree with the plan. I could give a try on the patch if you'd like.
> 
> Yes please, thanks!
> 
> 

As a KVM/x86 *feature*, playing with splitting and reconstructing large
pages have other potential user scenarios, e.g. for performance test
comparisons in a easier approach, not just for itlb_multihit mitigation.

On unaffected machines (ICX and later), nx_huge_pages is already "N",
and turning it into "never" doesn't help materially in the mitigation
implementation, but loses flexibility.

IMO, the real issue here is that the kernel thread "kvm-nx-lpage-
recovery" is created unconditionally. We also need to be aware of the
existence of this commit 084cc29f8bbb ("KVM: x86/MMU: Allow NX huge
pages to be disabled on a per-vm basis").

One of the technical proposals is to defer kvm_vm_create_worker_thread()
to kvm_mmu_create() or kvm_init_mmu(), based on 
kvm->arch.disable_nx_huge_pages, even until guest paging mode is enabled
on the first vcpu.

Is this step worth taking ?