Re: [PATCH v2 4/4] KVM: arm64: Refuse to run VCPU if the PMU doesn't match the physical CPU

[Marc Zyngier <maz@kernel.org>]

On Tue, 07 Dec 2021 14:17:56 +0000,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> Hi,
> 
> On Mon, Dec 06, 2021 at 05:02:23PM +0000, Alexandru Elisei wrote:
> > Userspace can assign a PMU to a VCPU with the KVM_ARM_VCPU_PMU_V3_SET_PMU
> > device ioctl. If the VCPU is scheduled on a physical CPU which has a
> > different PMU, the perf events needed to emulate a guest PMU won't be
> > scheduled in and the guest performance counters will stop counting. Treat
> > it as an userspace error and refuse to run the VCPU in this situation.
> > 
> > The VCPU is flagged as being scheduled on the wrong CPU in vcpu_load(), but
> > the flag is cleared when the KVM_RUN enters the non-preemptible section
> > instead of in vcpu_put(); this has been done on purpose so the error
> > condition is communicated as soon as possible to userspace, otherwise
> > vcpu_load() on the wrong CPU followed by a vcpu_put() would clear the flag.
> > 
> > Suggested-by: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> > I agonized for hours about the best name for the VCPU flag and the
> > accessors. If someone has a better idea, please tell me and I'll change
> > them.
> > 
> >  Documentation/virt/kvm/devices/vcpu.rst |  6 +++++-
> >  arch/arm64/include/asm/kvm_host.h       | 12 ++++++++++++
> >  arch/arm64/include/uapi/asm/kvm.h       |  3 +++
> >  arch/arm64/kvm/arm.c                    | 19 +++++++++++++++++++
> >  arch/arm64/kvm/pmu-emul.c               |  1 +
> >  5 files changed, 40 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/virt/kvm/devices/vcpu.rst b/Documentation/virt/kvm/devices/vcpu.rst
> > index c82be5cbc268..9ae47b7c3652 100644
> > --- a/Documentation/virt/kvm/devices/vcpu.rst
> > +++ b/Documentation/virt/kvm/devices/vcpu.rst
> > @@ -128,7 +128,11 @@ systems where there are at least two CPU PMUs on the system.
> >  
> >  Note that KVM will not make any attempts to run the VCPU on the physical CPUs
> >  associated with the PMU specified by this attribute. This is entirely left to
> > -userspace.
> > +userspace. However, attempting to run the VCPU on a physical CPU not supported
> > +by the PMU will fail and KVM_RUN will return with
> > +exit_reason = KVM_EXIT_FAIL_ENTRY and populate the fail_entry struct by setting
> > +hardare_entry_failure_reason field to KVM_EXIT_FAIL_ENTRY_CPU_UNSUPPORTED and
> > +the cpu field to the processor id.
> >  
> >  2. GROUP: KVM_ARM_VCPU_TIMER_CTRL
> >  =================================
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 2a5f7f38006f..0c453f2e48b6 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -385,6 +385,8 @@ struct kvm_vcpu_arch {
> >  		u64 last_steal;
> >  		gpa_t base;
> >  	} steal;
> > +
> > +	cpumask_var_t supported_cpus;
> >  };
> >  
> >  /* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
> > @@ -420,6 +422,7 @@ struct kvm_vcpu_arch {
> >  #define KVM_ARM64_EXCEPT_MASK		(7 << 9) /* Target EL/MODE */
> >  #define KVM_ARM64_DEBUG_STATE_SAVE_SPE	(1 << 12) /* Save SPE context if active  */
> >  #define KVM_ARM64_DEBUG_STATE_SAVE_TRBE	(1 << 13) /* Save TRBE context if active  */
> > +#define KVM_ARM64_ON_UNSUPPORTED_CPU	(1 << 14) /* Physical CPU not in supported_cpus */
> >  
> >  #define KVM_GUESTDBG_VALID_MASK (KVM_GUESTDBG_ENABLE | \
> >  				 KVM_GUESTDBG_USE_SW_BP | \
> > @@ -460,6 +463,15 @@ struct kvm_vcpu_arch {
> >  #define vcpu_has_ptrauth(vcpu)		false
> >  #endif
> >  
> > +#define vcpu_on_unsupported_cpu(vcpu)					\
> > +	((vcpu)->arch.flags & KVM_ARM64_ON_UNSUPPORTED_CPU)
> > +
> > +#define vcpu_set_on_unsupported_cpu(vcpu)				\
> > +	((vcpu)->arch.flags |= KVM_ARM64_ON_UNSUPPORTED_CPU)
> > +
> > +#define vcpu_clear_on_unsupported_cpu(vcpu)				\
> > +	((vcpu)->arch.flags &= ~KVM_ARM64_ON_UNSUPPORTED_CPU)
> > +
> >  #define vcpu_gp_regs(v)		(&(v)->arch.ctxt.regs)
> >  
> >  /*
> > diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> > index 1d0a0a2a9711..d49f714f48e6 100644
> > --- a/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/arch/arm64/include/uapi/asm/kvm.h
> > @@ -414,6 +414,9 @@ struct kvm_arm_copy_mte_tags {
> >  #define KVM_PSCI_RET_INVAL		PSCI_RET_INVALID_PARAMS
> >  #define KVM_PSCI_RET_DENIED		PSCI_RET_DENIED
> >  
> > +/* run->fail_entry.hardware_entry_failure_reason codes. */
> > +#define KVM_EXIT_FAIL_ENTRY_CPU_UNSUPPORTED	(1ULL << 0)
> > +
> >  #endif
> >  
> >  #endif /* __ARM_KVM_H__ */
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index e4727dc771bf..1124c3efdd94 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -327,6 +327,10 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >  
> >  	vcpu->arch.mmu_page_cache.gfp_zero = __GFP_ZERO;
> >  
> > +	if (!zalloc_cpumask_var(&vcpu->arch.supported_cpus, GFP_KERNEL))
> > +		return -ENOMEM;
> > +	cpumask_copy(vcpu->arch.supported_cpus, cpu_possible_mask);

Nit: can we just assign the cpu_possible_mask pointer instead, and
only perform the allocation when assigning a specific PMU?

> > +
> >  	/* Set up the timer */
> >  	kvm_timer_vcpu_init(vcpu);
> >  
> > @@ -354,6 +358,7 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
> >  	if (vcpu->arch.has_run_once && unlikely(!irqchip_in_kernel(vcpu->kvm)))
> >  		static_branch_dec(&userspace_irqchip_in_use);
> >  
> > +	free_cpumask_var(vcpu->arch.supported_cpus);
> >  	kvm_mmu_free_memory_cache(&vcpu->arch.mmu_page_cache);
> >  	kvm_timer_vcpu_terminate(vcpu);
> >  	kvm_pmu_vcpu_destroy(vcpu);
> > @@ -432,6 +437,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >  	if (vcpu_has_ptrauth(vcpu))
> >  		vcpu_ptrauth_disable(vcpu);
> >  	kvm_arch_vcpu_load_debug_state_flags(vcpu);
> > +
> > +	if (!cpumask_test_cpu(smp_processor_id(), vcpu->arch.supported_cpus))
> > +		vcpu_set_on_unsupported_cpu(vcpu);
> >  }
> >  
> >  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > @@ -822,6 +830,17 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> >  		 */
> >  		preempt_disable();
> >  
> > +		if (unlikely(vcpu_on_unsupported_cpu(vcpu))) {
> > +			vcpu_clear_on_unsupported_cpu(vcpu);
> > +			run->exit_reason = KVM_EXIT_FAIL_ENTRY;
> > +			run->fail_entry.hardware_entry_failure_reason
> > +				= KVM_EXIT_FAIL_ENTRY_CPU_UNSUPPORTED;
> > +			run->fail_entry.cpu = smp_processor_id();
>

Can you move this hunk to kvm_vcpu_exit_request()? It certainly would
fit better there, as we have checks for other exit reasons to
userspace.

> I just realised that this is wrong for the same reason that KVM doesn't
> clear the unsupported CPU flag on vcpu_put: a vcpu_put/load that happened
> after the vcpu_load that set the flag and before preemption is disabled
> could mean that now the thread is executing on a different physical CPU
> than the physical CPU that caused the flag to be set. To make things worse,
> this CPU might even be in supported_cpus, which would be extremely
> confusing for someone trying to descipher what went wrong.
> 
> I see three solutions here:
> 
> 1. Drop setting the fail_entry.cpu field.
> 
> 2. Make vcpu_put clear the flag, which means that if the flag is set here
> then the VCPU is definitely executing on the wrong physical CPU and
> smp_processor_id() will be useful.

This looks reasonable to me.

> 
> 3. Carry the unsupported CPU ID information in a new field in struct
> kvm_vcpu_arch.
> 
> I honestly don't have a preference. Maybe slightly towards solution number
> 2, as it makes the code symmetrical and removes the subtletly around when
> the VCPU flag is cleared. But this would be done at the expense of
> userspace possibly finding out a lot later (or never) that something went
> wrong.

I don't really get your argument about "userspace possibly finding out
a lot later...". Yes, if the vcpu gets migrated to a 'good' CPU after
a sequence of put/load, userspace will be lucky. But that's the rule
of the game. If userspace pins the vcpu to the wrong CPU type, then
the information will be consistent.

	M.

-- 
Without deviation from the norm, progress is not possible.

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