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@bai... 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.

Tests performed:

* Confirmed KVM_CREATE_VM latency goes down to less than 1ms * Quickly booted a simple guest with kvmtool

[1] https://lore.kernel.org/stable/cover.1693593288.git.luizcap@amazon.com/

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 6eaa3d6994ae..11c050f40d82 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); @@ -7164,6 +7192,9 @@ 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);