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Message-ID: <7b6ce80e-7f1f-11cd-8bde-8d8fa9fd7e1d@redhat.com>
Date: Thu, 3 Nov 2022 16:23:12 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
Marc Zyngier <maz@...nel.org>,
Huacai Chen <chenhuacai@...nel.org>,
Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
Anup Patel <anup@...infault.org>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Janosch Frank <frankja@...ux.ibm.com>,
Claudio Imbrenda <imbrenda@...ux.ibm.com>,
Matthew Rosato <mjrosato@...ux.ibm.com>,
Eric Farman <farman@...ux.ibm.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: James Morse <james.morse@....com>,
Alexandru Elisei <alexandru.elisei@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Oliver Upton <oliver.upton@...ux.dev>,
Atish Patra <atishp@...shpatra.org>,
David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
kvmarm@...ts.cs.columbia.edu, linux-mips@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
linux-kernel@...r.kernel.org,
Isaku Yamahata <isaku.yamahata@...el.com>,
Fabiano Rosas <farosas@...ux.ibm.com>,
Michael Ellerman <mpe@...erman.id.au>,
Chao Gao <chao.gao@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH 39/44] KVM: Drop kvm_count_lock and instead protect
kvm_usage_count with kvm_lock
On 11/3/22 00:19, Sean Christopherson wrote:
> +- kvm_lock is taken outside kvm->mmu_lock
Not surprising since one is a mutex and one is an rwlock. :) You can
drop this hunk as well as the "Opportunistically update KVM's locking
documentation" sentence in the commit message.
> - vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock
>
> - kvm->arch.mmu_lock is an rwlock. kvm->arch.tdp_mmu_pages_lock and
> @@ -216,15 +220,11 @@ time it will be set using the Dirty tracking mechanism described above.
> :Type: mutex
> :Arch: any
> :Protects: - vm_list
> -
> -``kvm_count_lock``
> -^^^^^^^^^^^^^^^^^^
> -
> -:Type: raw_spinlock_t
> -:Arch: any
> -:Protects: - hardware virtualization enable/disable
> -:Comment: 'raw' because hardware enabling/disabling must be atomic /wrt
> - migration.
> + - kvm_usage_count
> + - hardware virtualization enable/disable
> + - module probing (x86 only)
What do you mean exactly by "module probing"? Is it anything else than
what is serialized by vendor_module_lock?
Paolo
> +:Comment: KVM also disables CPU hotplug via cpus_read_lock() during
> + enable/disable.
>
> ``kvm->mn_invalidate_lock``
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 4e765ef9f4bd..c8d92e6c3922 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -100,7 +100,6 @@ EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
> */
>
> DEFINE_MUTEX(kvm_lock);
> -static DEFINE_RAW_SPINLOCK(kvm_count_lock);
> LIST_HEAD(vm_list);
>
> static cpumask_var_t cpus_hardware_enabled;
> @@ -5028,9 +5027,10 @@ static void hardware_enable_nolock(void *junk)
>
> static int kvm_online_cpu(unsigned int cpu)
> {
> + unsigned long flags;
> int ret = 0;
>
> - raw_spin_lock(&kvm_count_lock);
> + mutex_lock(&kvm_lock);
> /*
> * Abort the CPU online process if hardware virtualization cannot
> * be enabled. Otherwise running VMs would encounter unrecoverable
> @@ -5039,13 +5039,16 @@ static int kvm_online_cpu(unsigned int cpu)
> if (kvm_usage_count) {
> WARN_ON_ONCE(atomic_read(&hardware_enable_failed));
>
> + local_irq_save(flags);
> hardware_enable_nolock(NULL);
> + local_irq_restore(flags);
> +
> if (atomic_read(&hardware_enable_failed)) {
> atomic_set(&hardware_enable_failed, 0);
> ret = -EIO;
> }
> }
> - raw_spin_unlock(&kvm_count_lock);
> + mutex_unlock(&kvm_lock);
> return ret;
> }
>
> @@ -5061,10 +5064,13 @@ static void hardware_disable_nolock(void *junk)
>
> static int kvm_offline_cpu(unsigned int cpu)
> {
> - raw_spin_lock(&kvm_count_lock);
> - if (kvm_usage_count)
> + mutex_lock(&kvm_lock);
> + if (kvm_usage_count) {
> + preempt_disable();
> hardware_disable_nolock(NULL);
> - raw_spin_unlock(&kvm_count_lock);
> + preempt_enable();
> + }
> + mutex_unlock(&kvm_lock);
> return 0;
> }
>
> @@ -5079,9 +5085,11 @@ static void hardware_disable_all_nolock(void)
>
> static void hardware_disable_all(void)
> {
> - raw_spin_lock(&kvm_count_lock);
> + cpus_read_lock();
> + mutex_lock(&kvm_lock);
> hardware_disable_all_nolock();
> - raw_spin_unlock(&kvm_count_lock);
> + mutex_unlock(&kvm_lock);
> + cpus_read_unlock();
> }
>
> static int hardware_enable_all(void)
> @@ -5097,7 +5105,7 @@ static int hardware_enable_all(void)
> * Disable CPU hotplug to prevent scenarios where KVM sees
> */
> cpus_read_lock();
> - raw_spin_lock(&kvm_count_lock);
> + mutex_lock(&kvm_lock);
>
> kvm_usage_count++;
> if (kvm_usage_count == 1) {
> @@ -5110,7 +5118,7 @@ static int hardware_enable_all(void)
> }
> }
>
> - raw_spin_unlock(&kvm_count_lock);
> + mutex_unlock(&kvm_lock);
> cpus_read_unlock();
>
> return r;
> @@ -5716,6 +5724,15 @@ static void kvm_init_debug(void)
>
> static int kvm_suspend(void)
> {
> + /*
> + * Secondary CPUs and CPU hotplug are disabled across the suspend/resume
> + * callbacks, i.e. no need to acquire kvm_lock to ensure the usage count
> + * is stable. Assert that kvm_lock is not held as a paranoid sanity
> + * check that the system isn't suspended when KVM is enabling hardware.
> + */
> + lockdep_assert_not_held(&kvm_lock);
> + lockdep_assert_irqs_disabled();
> +
> if (kvm_usage_count)
> hardware_disable_nolock(NULL);
> return 0;
> @@ -5723,10 +5740,11 @@ static int kvm_suspend(void)
>
> static void kvm_resume(void)
> {
> - if (kvm_usage_count) {
> - lockdep_assert_not_held(&kvm_count_lock);
> + lockdep_assert_not_held(&kvm_lock);
> + lockdep_assert_irqs_disabled();
> +
> + if (kvm_usage_count)
> hardware_enable_nolock(NULL);
> - }
> }
>
> static struct syscore_ops kvm_syscore_ops = {
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