Directly reserve an interrupt IDT entry for Hyper-V

[Neel Natu]

Hi John,

On Wed, Aug 20, 2014 at 8:31 AM, John Baldwin <jhb at freebsd.org> wrote:
> On Wednesday, August 20, 2014 9:31:54 am Roger Pau Monné wrote:
>> On 20/08/14 11:19, Wei Hu wrote:
>> > Hello,
>> >
>> > Sending to Xen, drivers and virtualization mailing lists since this might
> be of interest to the folks on these aliases.
>> >
>> > I am working for Microsoft to improve the performance of FreeBSD running
> on Hyper-V. Right now I am adding a feature in the vmbus driver which could
> handle the host-guest channel communications on all vCPUs simultaneously. In
> order to achieve this, the hypervisor will send same interrupt concurrently on
> all the vCPUs. The traditional way on FreeBSD to set up interrupt handling for
> devicse, such as calling bus_alloc_resource() to reserve an IRQ line, and then
> calling bus_setup_intr() to create a vector, doesn't seem to work in this
> case. It seems if the interrupt is routed via legacy IRQ, it can only be
> active on one vCPU at a time. In order to allow the same interrupt to be
> handled on all vCPUs concurrently, all I need is an IDT entry, not an IRQ
> line.
>> >
>> > I checked current FreeBSD code. It looks to me Xen directly uses the
> vector number IDT_EVTCHN (0x93) to achieve the same purpose. I am proposing
> both Xen and Hyper-V share this same vector. Following is a little bit detail
> of my proposal for the changes in the current kernel.
>> >
>> >
>> > 1.       In machdep.c:
>> >
>> >  #ifdef XENHVM
>> >
>> >         setidt(IDT_EVTCHN, &IDTVEC(xen_intr_upcall), SDT_SYSIGT, SEL_UPL,
> 0);
>> >
>> > #else
>> >
>> >         setidt(IDT_EVTCHN, &IDTVEC(hv_vmbus_intr), SDT_SYSIGT, SEL_UPL,
> 0);
>> >
>> > #endif
>> >
>> > 2.       Apic_vector.S
>> >
>> > Add IDTVEC(hv_vmbus_intr) to call Hyper-V vmbus interrupt service routine.
>> >
>> > Any  thoughts, objections and feedbacks are all welcome.
>>
>> Hello,
>>
>> I don't think using the same IDT vector is the right approach, I would
>> just pick a different IDT vector and use that for Hyper-V. Using the
>> same IDT vector (like your suggestion above) would prevent shipping a
>> kernel with with both Hyper-V and Xen support (like it's done now in
>> GENERIC).
>>
>> Roger.
>
> Hmm, can't you make this a runtime check to only call setidt() if you detect
> you are under the appropriate hypervisor?
>
> Also, bhyve currently has a hackish way of requesting a free IDT slot.
> Perhaps it would be best if I added little API to reserve an IDT slot assuming
> that callers could accept a dynamic IDT vector rather than a static one.
>

Yup, it'll be good to get rid of vmm_ipi.c.

best
Neel

> --
> John Baldwin
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