Re: [PATCH] KVM: x86/mmu: Add "never" option to allow sticky disabling of nx_huge_pages

[Luiz Capitulino]

On Thu, Jun 01, 2023 at 05:58:59PM -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.
> 
> 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.

Tested-by: Luiz Capitulino <luizcap@xxxxxxxxxx>

Without this patch I can see the 100ms+ latencies on KVM_CREATE_VM even
with a single VM.

Just run a VM with with strace -T and grep for KVM_CREATE_VM. When using
kvmtool I get (latency in seconds - kernel HEAD is a4d7d70112):

  ioctl(3, KVM_CREATE_VM, 0)              = 4 <0.023567>
  
  ioctl(3, KVM_CREATE_VM, 0)              = 4 <0.076709>
  
  ioctl(3, KVM_CREATE_VM, 0)              = 4 <0.109109>

With this patch and nx_huge_page=never:

  ioctl(3, KVM_CREATE_VM, 0)              = 4 <0.000518>
  
  ioctl(3, KVM_CREATE_VM, 0)              = 4 <0.000495>
  
  ioctl(3, KVM_CREATE_VM, 0)              = 4 <0.000513>

Now, I debugged down the single VM case before seeing this patch and it
can be avoided by building the kernel with CONFIG_CGROUP_FAVOR_DYNMODS=y
or mounting the cgroup v2 mount point with the favordynmods mount option.

This is because the high latency is coming from a call to
cgroup_attach_task_all() in:

kvm_vm_worker_thread()
  cgroup_attach_task_all()
    percpu_down_write(&cgroup_threadgroup_rwsem)
      /* calls synchronize_rcu() */

This happens while kvm_vm_create_worker_thread() is waiting on a completion.
See commit 6a010a49b63a for more information.

This patch is preferable because the favordynmods solution has a trade-off.
However, why don't we make nx_huge_pages=never the default behavior if the
CPU is not vulnerable? If there are concerns about not being able to restart
the worker thread, then maybe we could make this a .config option?

- Luiz


> 
> Reported-by: Li RongQing <lirongqing@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/1679555884-32544-1-git-send-email-lirongqing@xxxxxxxxx
> Cc: Yong He <zhuangel570@xxxxxxxxx>
> Cc: Robert Hoo <robert.hoo.linux@xxxxxxxxx>
> Cc: Kai Huang <kai.huang@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 41 ++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 36 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index c8961f45e3b1..2ed38916b904 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -58,6 +58,8 @@
>  
>  extern bool itlb_multihit_kvm_mitigation;
>  
> +static bool nx_hugepage_mitigation_hard_disabled;
> +
>  int __read_mostly nx_huge_pages = -1;
>  static uint __read_mostly nx_huge_pages_recovery_period_ms;
>  #ifdef CONFIG_PREEMPT_RT
> @@ -67,12 +69,13 @@ static uint __read_mostly nx_huge_pages_recovery_ratio = 0;
>  static uint __read_mostly nx_huge_pages_recovery_ratio = 60;
>  #endif
>  
> +static int get_nx_huge_pages(char *buffer, const struct kernel_param *kp);
>  static int set_nx_huge_pages(const char *val, const struct kernel_param *kp);
>  static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel_param *kp);
>  
>  static const struct kernel_param_ops nx_huge_pages_ops = {
>  	.set = set_nx_huge_pages,
> -	.get = param_get_bool,
> +	.get = get_nx_huge_pages,
>  };
>  
>  static const struct kernel_param_ops nx_huge_pages_recovery_param_ops = {
> @@ -6844,6 +6847,14 @@ static void mmu_destroy_caches(void)
>  	kmem_cache_destroy(mmu_page_header_cache);
>  }
>  
> +static int get_nx_huge_pages(char *buffer, const struct kernel_param *kp)
> +{
> +	if (nx_hugepage_mitigation_hard_disabled)
> +		return sprintf(buffer, "never\n");
> +
> +	return param_get_bool(buffer, kp);
> +}
> +
>  static bool get_nx_auto_mode(void)
>  {
>  	/* Return true when CPU has the bug, and mitigations are ON */
> @@ -6860,15 +6871,29 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
>  	bool old_val = nx_huge_pages;
>  	bool new_val;
>  
> +	if (nx_hugepage_mitigation_hard_disabled)
> +		return -EPERM;
> +
>  	/* In "auto" mode deploy workaround only if CPU has the bug. */
> -	if (sysfs_streq(val, "off"))
> +	if (sysfs_streq(val, "off")) {
>  		new_val = 0;
> -	else if (sysfs_streq(val, "force"))
> +	} else if (sysfs_streq(val, "force")) {
>  		new_val = 1;
> -	else if (sysfs_streq(val, "auto"))
> +	} else if (sysfs_streq(val, "auto")) {
>  		new_val = get_nx_auto_mode();
> -	else if (kstrtobool(val, &new_val) < 0)
> +	} else if (sysfs_streq(val, "never")) {
> +		new_val = 0;
> +
> +		mutex_lock(&kvm_lock);
> +		if (!list_empty(&vm_list)) {
> +			mutex_unlock(&kvm_lock);
> +			return -EBUSY;
> +		}
> +		nx_hugepage_mitigation_hard_disabled = true;
> +		mutex_unlock(&kvm_lock);
> +	} else if (kstrtobool(val, &new_val) < 0) {
>  		return -EINVAL;
> +	}
>  
>  	__set_nx_huge_pages(new_val);
>  
> @@ -7006,6 +7031,9 @@ static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel
>  	uint old_period, new_period;
>  	int err;
>  
> +	if (nx_hugepage_mitigation_hard_disabled)
> +		return -EPERM;
> +
>  	was_recovery_enabled = calc_nx_huge_pages_recovery_period(&old_period);
>  
>  	err = param_set_uint(val, kp);
> @@ -7161,6 +7189,9 @@ int kvm_mmu_post_init_vm(struct kvm *kvm)
>  {
>  	int err;
>  
> +	if (nx_hugepage_mitigation_hard_disabled)
> +		return 0;
> +
>  	err = kvm_vm_create_worker_thread(kvm, kvm_nx_huge_page_recovery_worker, 0,
>  					  "kvm-nx-lpage-recovery",
>  					  &kvm->arch.nx_huge_page_recovery_thread);
> 
> base-commit: 39428f6ea9eace95011681628717062ff7f5eb5f
> -- 
> 2.41.0.rc2.161.g9c6817b8e7-goog
>