On 2016/4/29 12:59, Jan Kiszka wrote:
On 2016-04-29 04:09, Yang Zhang wrote:
On 2016/4/28 23:32, Radim Krčmář wrote:
2016-04-28 08:54+0200, Jan Kiszka:
On 2016-04-28 03:11, Yang Zhang wrote:
On 2016/4/27 17:45, Jan Kiszka wrote:
On 2016-04-27 11:39, Yang Zhang wrote:
I mean in Tianyu's case, if he doesn't care about to deliver external
interrupt to CPU >255, IR is not required.
What matters is the guest OS. See my other reply on this why this
doesn't work, even for Linux.
Since there only few devices in his case, set the irq affinity manually
is enough.
You could configure non-IPIs to work, but we want to create options that
are hard to break.
Ah, wait - are we talking about emulating the Xeon Phi architecture in
QEMU, accelerated by KVM?
Knights Landing will also be manufactured as a CPU, hopefully without
many peculiarities.
I think we are talking about extending KVM's IR-less x2APIC, when
standard x2APIC is the future.
Yes, Since IR is only useful for the external device, and 255 CPUs is
enough to handle the interrupts from external devices. Besides, i think
virtual VT-d will bring extra performance impaction for devices, so if
IR-less X2APIC also works well with more than 255 VCPUs, maybe extending
KVM with IR-less x2apic is not a bad idea.
IR-less x2APIC for guest architectures that are expected to provide IR
remains a bad idea, at least until we have hard numbers what this
suspected performance impact actually is. Unless you update IRQ
affinities an insane rates, the impact should not be relevant because
remapping results are cached (for the irqfd hot-path) or you are already
taking the long way (userspace device emulation).
I think it is not only interrupt. There must have the DMAR emulation and
the cost for DMA is heavy in VM(DMA operations are very frequently). I
cannot remember whether there are strong dependency in hardware between
DMAR and IR(I know IR is relying on QI). Even hardware dependency is ok,
is it ok for OS running in hardware with IR but without DMAR?
--
best regards
yang
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