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

[Robert Hoo]

On 6/2/2023 8:58 AM, 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.
>
> 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;
> +	}

IIUC, (Initially) "auto_off"/"off" --> create some VM --> turn to "never", 
the created VMs still have those kthreads, but can never be used, until 
destroyed with VM.

Is this designed/accepted pattern?

>   	__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