On 07/04/2020 17:50, Julien Grall wrote:
On 07/04/2020 17:16, George Dunlap wrote:
On Apr 6, 2020, at 7:47 PM, Julien Grall <jul...@xen.org> wrote:
On 06/04/2020 18:58, George Dunlap wrote:
On Apr 3, 2020, at 9:27 PM, Julien Grall
<julien.grall....@gmail.com> wrote:
On Fri, 3 Apr 2020 at 20:41, Stefano Stabellini
<sstabell...@kernel.org> wrote:
On Thu, 2 Apr 2020, Julien Grall wrote:
As we discussed on Tuesday, the cost for other vCPUs is only
going to be a
trap to the hypervisor and then back again. The cost is likely
smaller than
receiving and forwarding an interrupt.
You actually agreed on this analysis. So can you enlighten me as
to why
receiving an interrupt is a not problem for latency but this is?
My answer was that the difference is that an operating system can
disable interrupts, but it cannot disable receiving this special IPI.
An OS can *only* disable its own interrupts, yet interrupts will still
be received by Xen even if the interrupts are masked at the processor
(e.g local_irq_disable()).
You would need to disable interrupts one by one as the GIC level (use
ICENABLER) in order to not receive any interrupts. Yet, Xen may still
receive interrupts for operational purposes (e.g serial, console,
maintainance IRQ...). So trap will happen.
I think Stefano’s assertion is that the users he has in mind will be
configuring the system such that RT workloads get a minimum number
of hypervisor-related interrupts possible. On a 4-core system, you
could have non-RT workloads running on cores 0-1, and RT workloads
running with the NULL scheduler on cores 2-3. In such a system,
you’d obviously arrange that serial and maintenance IRQs are
delivered to cores 0-1.
Well maintenance IRQs are per pCPU so you can't route to another one...
But, I think you missed my point that local_irq_disable() from the
guest will not prevent the hypervisor to receive interrupts *even*
the one routed to the vCPU itself. They will just not be delivered to
the guest context until local_irq_enable() is called.
My understanding, from Stefano was that what his customers are
concerned about is the time between the time a physical IRQ is
delivered to the guest and the time the guest OS can respond
appropriately. The key thing here isn’t necessarily speed, but
predictability — system designers need to know that, with a high
probability, their interrupt routines will complete within X amount of
cycles.
Further interrupts delivered to a guest are not a problem in this
scenario, if the guest can disable them until the critical IRQ has
been handled.
You keep saying a guest can disable interrupts, but it can't do it via
local_irq_disable(). So what method are you thinking? Disabling at the
GIC level? That is involving traps and most likely not going to help
with predictability...
Just to clear I meant interrupts to be received by Xen including the one
routed to that vCPU.
Cheers,
--
Julien Grall
