From 30c60dda219ddda0bc6ff6ac55d493d9db8be4fa Mon Sep 17 00:00:00 2001 From: Oliver Upton Date: Tue, 6 Jun 2023 18:48:14 +0000 Subject: [PATCH] KVM: arm64: Use raw_smp_processor_id() in kvm_pmu_probe_armpmu() Sebastian reports that commit 1c913a1c35aa ("KVM: arm64: Iterate arm_pmus list to probe for default PMU") introduced the following splat with CONFIG_DEBUG_PREEMPT enabled: [70506.110187] BUG: using smp_processor_id() in preemptible [00000000] code: qemu-system-aar/3078242 [70506.119077] caller is debug_smp_processor_id+0x20/0x30 [70506.124229] CPU: 129 PID: 3078242 Comm: qemu-system-aar Tainted: G W 6.4.0-rc5 #25 [70506.133176] Hardware name: GIGABYTE R181-T92-00/MT91-FS4-00, BIOS F34 08/13/2020 [70506.140559] Call trace: [70506.142993] dump_backtrace+0xa4/0x130 [70506.146737] show_stack+0x20/0x38 [70506.150040] dump_stack_lvl+0x48/0x60 [70506.153704] dump_stack+0x18/0x28 [70506.157007] check_preemption_disabled+0xe4/0x108 [70506.161701] debug_smp_processor_id+0x20/0x30 [70506.166046] kvm_arm_pmu_v3_set_attr+0x460/0x628 [70506.170662] kvm_arm_vcpu_arch_set_attr+0x88/0xd8 [70506.175363] kvm_arch_vcpu_ioctl+0x258/0x4a8 [70506.179632] kvm_vcpu_ioctl+0x32c/0x6b8 [70506.183465] __arm64_sys_ioctl+0xb4/0x100 [70506.187467] invoke_syscall+0x78/0x108 [70506.191205] el0_svc_common.constprop.0+0x4c/0x100 [70506.195984] do_el0_svc+0x34/0x50 [70506.199287] el0_svc+0x34/0x108 [70506.202416] el0t_64_sync_handler+0xf4/0x120 [70506.206674] el0t_64_sync+0x194/0x198 Fix the issue by using the raw variant that bypasses the debug assertion. While at it, stick all of the nuance and UAPI baggage into a comment for posterity. Fixes: 1c913a1c35aa ("KVM: arm64: Iterate arm_pmus list to probe for default PMU") Reported-by: Sebastian Ott Signed-off-by: Oliver Upton Signed-off-by: Marc Zyngier Link: https://lore.kernel.org/r/20230606184814.456743-1-oliver.upton@linux.dev --- arch/arm64/kvm/pmu-emul.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c index 491ca7eb2a4c6e..56065097247873 100644 --- a/arch/arm64/kvm/pmu-emul.c +++ b/arch/arm64/kvm/pmu-emul.c @@ -700,7 +700,25 @@ static struct arm_pmu *kvm_pmu_probe_armpmu(void) mutex_lock(&arm_pmus_lock); - cpu = smp_processor_id(); + /* + * It is safe to use a stale cpu to iterate the list of PMUs so long as + * the same value is used for the entirety of the loop. Given this, and + * the fact that no percpu data is used for the lookup there is no need + * to disable preemption. + * + * It is still necessary to get a valid cpu, though, to probe for the + * default PMU instance as userspace is not required to specify a PMU + * type. In order to uphold the preexisting behavior KVM selects the + * PMU instance for the core where the first call to the + * KVM_ARM_VCPU_PMU_V3_CTRL attribute group occurs. A dependent use case + * would be a user with disdain of all things big.LITTLE that affines + * the VMM to a particular cluster of cores. + * + * In any case, userspace should just do the sane thing and use the UAPI + * to select a PMU type directly. But, be wary of the baggage being + * carried here. + */ + cpu = raw_smp_processor_id(); list_for_each_entry(entry, &arm_pmus, entry) { tmp = entry->arm_pmu;