Directly reserve an interrupt IDT entry for Hyper-V

[Roger Pau Monné]

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.