Directly reserve an interrupt IDT entry for Hyper-V

[John Baldwin]

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.

-- 
John Baldwin