Re: [PATCH 0/7] Consolidate vcpu ioctl locking

[Avi Kivity <avi-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 05/15/2010 09:21 AM, Alexander Graf wrote:
>
>> On x86 eflags.if is freely changeable by the guest, so if we want to queue an interrupt we have to IPI the vcpu to force it out of guest mode, so we can inspect eflags.  This means the vcpu thread has to be interrupted one way or another.
>>
>> The tpr (really ppr) is even more problematic as it is maintained in userspace, not in the kernel (for non-kernel-irqchip).  It could in theory be inspected by another thread, but we wouldn't gain anything by it due to the requirement to IPI.
>>      
> Hrm right. On PPC we trap on every MSR change, so we get notified when interrupts are enabled again. But isn't that what VINTR intercepts are there for? Just add the  lowest active TPR value to the KVM_INTERRUPT ioctl and then wait until the guest is ready to take it.
>    

Yes, and we do that when using in-kernel-irqchip.  With userspace apic, 
the tpr is maintained in userspace and the kernel has no visibility into it.

We could have changed moved the tpr/ppr into the kernel for userspace 
irqchip, but there's not much point now.

>>> void kvmppc_book3s_queue_irqprio(struct kvm_vcpu *vcpu, unsigned int vec)
>>> {
>>>     vcpu->stat.queue_intr++;
>>>
>>>     set_bit(kvmppc_book3s_vec2irqprio(vec),
>>> &vcpu->arch.pending_exceptions);
>>> #ifdef EXIT_DEBUG
>>>     printk(KERN_INFO "Queueing interrupt %x\n", vec);
>>> #endif
>>> }
>>>        
>> Isn't this missing an IPI if the vcpu is in guest mode?
>>      
> Yes, it is :). At least with qemu we're 100% sure we're not in VCPU_RUN when an interrupt gets injected, as the injection happens in kvm_arch_pre_run.
>    

That means you never inject an interrupt from the iothread (or from a 
different vcpu thread)?

If that's the case we might make it part of the API and require the 
ioctl to be issued from the vcpu thread.  We'd still be left with the 
s390 exception.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.