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

Gleb Natapov gleb at redhat.com
Wed Jan 16 05:40:55 EST 2013

On Tue, Jan 15, 2013 at 05:25:13PM +0100, Alexander Graf wrote:
> On 01/15/2013 04:17 PM, Gleb Natapov wrote:
> >On Tue, Jan 15, 2013 at 02:04:47PM +0000, Peter Maydell wrote:
> >>On 15 January 2013 12:52, Gleb Natapov<gleb at 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 at 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.
> >>
> >OK. I see. So basically Avi saw KVM_INTERRUPT as an oddball interface
> >required only for APIC emulation in userspace. It is used for PIC also,
> >where this is not strictly needed, but this is for historical reasons
> >(KVM_IRQ_LINE was introduces late and it is GSI centric on x86).
> >
> >Thank you for the pointer.
> Yeah, please keep in mind that KVM_INTERRUPT is not a unified
> interface either. In fact, it is asynchronous on PPC :). And it's
> called KVM_S390_INTERRUPT on s390 and also asynchronous. X86 is the
> oddball here.
KVM_INTERRUPT needs vcpu fd to be issues. Usually such ioctls are
issued only by vcpu thread which makes them synchronous and vcpu_load()
synchronise them anyway if the rule is not met. And sure enough those
KVM_S390_INTERRUPT/KVM_INTERRUPT are special cased in kvm_vcpu_ioctl()
to not call vcpu_load(), sigh :(

There was an idea to change vcpu ioctls to kvm syscall which would have
made it impossible to use KVM_INTERRUPT asynchronously.

> But I don't care whether we call the ioctl to steer CPU interrupt
> the code makes it obvious what is happening.
Some consistency would be nice though. You do not always look at the
kernel code when you read userspace code and iothread calling KVM_INTERRUPT
would have made me suspicious.


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