Re: [kvmarm] [PATCH v5 06/14] KVM: ARM: Inject IRQs and FIQs from userspace

Christoffer Dall
2013-01-15T14:40:13Z

On Tue, Jan 15, 2013 at 9:04 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
> On 15 January 2013 12:52, Gleb Natapov <gleb@redhat.com> wrote:
>> On Tue, Jan 15, 2013 at 12:15:01PM +0000, Peter Maydell wrote:
>>> On 15 January 2013 09:56, Gleb Natapov <gleb@redhat.com> wrote:
>>> >> ARM can signal an interrupt either at the CPU level, or at the in-kernel irqchip
>>> > CPU level interrupt should use KVM_INTERRUPT instead.
>>>
>>> No, that would be wrong. KVM_INTERRUPT is for interrupts which must be
>>> delivered synchronously to the CPU. KVM_IRQ_LINE is for interrupts which
>>> can be fed to the kernel asynchronously. It happens that on x86 "must be
>>> delivered synchronously" and "not going to in kernel irqchip" are the same, but
>>> this isn't true for other archs. For ARM all our interrupts can be fed
>>> to the kernel asynchronously, and so we use KVM_IRQ_LINE in all
>>> cases.
>
>> I do no quite understand what you mean by synchronously and
>> asynchronously.
>
> Synchronously: the vcpu has to be stopped and userspace then
> feeds in the interrupt to be taken when the guest is resumed.
> Asynchronously: any old thread can tell the kernel there's an
> interrupt, and the guest vcpu then deals with it when needed
> (the vcpu thread may leave the guest but doesn't come out of
> the host kernel to qemu).
>
>> The difference between KVM_INTERRUPT and KVM_IRQ_LINE line
>> is that former is used when destination cpu is known to userspace later
>> is used when kernel code is involved in figuring out the destination.
>
> This doesn't match up with Avi's explanation at all.
>
>> The
>> injections themselves are currently synchronous for both of them on x86
>> and ARM. i.e vcpu is kicked out from guest mode when interrupt need to
>> be injected into a guest and vcpu state is changed to inject interrupt
>> during next guest entry. In the near feature x86 will be able to inject
>> interrupt without kicking vcpu out from the guest mode does ARM plan to
>> do the same? For GIC interrupts or for IRQ/FIQ or for both?
>>
>>> There was a big discussion thread about this on kvm and qemu-devel last
>>> July (and we cleaned up some of the QEMU code to not smoosh together
>>> all these different concepts under "do I have an irqchip or not?").
>> Do you have a pointer?
>
> http://lists.gnu.org/archive/html/qemu-devel/2012-07/msg02460.html
> and there was a later longer (but less clear) thread which included
> this mail from Avi:
> http://lists.gnu.org/archive/html/qemu-devel/2012-07/msg02872.html
> basically explaining that the reason for the weird synchronous
> KVM_INTERRUPT API is that it's emulating a weird synchronous
> hardware interface which is specific to x86. ARM doesn't have
> a synchronous interface in the same way, so it's much more
> straightforward to use KVM_IRQ_LINE.
>
Also, this code has been reviewed numerous times by the KVM community
and as Peter points out has also been discussed in detail.

Could we please not change this API in the last second?

-Christoffer