Re: [PATH 6.4.y] KVM: x86/mmu: Add "never" option to allow sticky disabling of nx_huge_pages

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 05, 2023, Luiz Capitulino wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> Commit 0b210faf337314e4bc88e796218bc70c72a51209 upstream.
> 
> 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.
> 
> E.g. in FaaS use cases, the guest kernel is trusted and the host may
> create 100+ VMs per logical CPU, which can result in 100ms+ latencies when
> a burst of VMs is created.
> 
> Reported-by: Li RongQing <lirongqing@baidu.com>
> Closes: https://lore.kernel.org/all/1679555884-32544-1-git-send-email-lirongqing@baidu.com
> Cc: Yong He <zhuangel570@gmail.com>
> Cc: Robert Hoo <robert.hoo.linux@gmail.com>
> Cc: Kai Huang <kai.huang@intel.com>
> Reviewed-by: Robert Hoo <robert.hoo.linux@gmail.com>
> Acked-by: Kai Huang <kai.huang@intel.com>
> Tested-by: Luiz Capitulino <luizcap@amazon.com>
> Reviewed-by: Li RongQing <lirongqing@baidu.com>
> Link: https://lore.kernel.org/r/20230602005859.784190-1-seanjc@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Luiz Capitulino <luizcap@amazon.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 41 ++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 36 insertions(+), 5 deletions(-)
> 
> I submitted this backport for 6.1.y[1] but we agreed that having it for 6.4.y
> is desirable to allow upgrade path.

Heh, I would have personally just let 6.4 suffer, but since you went through the
effort:

Acked-by: Sean Christopherson <seanjc@google.com>