Re: Routing physical interrupts to EL1

[Julien Grall <julien.grall@arm.com>]

Hello,

Please configure your e-mail client to quote properly (i.e with >).

On 02/08/2018 20:14, Saeed Mirzamohammadi wrote:
> Sorry for the late reply.
> 
> On Sat, Jul 7, 2018 at 2:25 PM, Julien Grall <julien.grall@arm.com 
> <mailto:julien.grall@arm.com>> wrote:
> 
>     Hello,
> 
>     On 07/07/2018 08:32 PM, Saeed Mirzamohammadi wrote:
> 
>         Thanks for your detailed reply.
> 
>         On Fri, Jul 6, 2018 at 6:13 AM, Julien Grall
>         <julien.grall@arm.com <mailto:julien.grall@arm.com>
>         <mailto:julien.grall@arm.com <mailto:julien.grall@arm.com>>> wrote:
> 
> 
> 
>              On 06/07/18 04:51, Saeed Mirzamohammadi wrote:
> 
>                  Hi,
> 
> 
>              Hello,
> 
>                  I'm trying to route all the physical interrupts to the
>         guest
>                  domain rather than being trapped in the Xen. I would
>         like to
>                  know what is the right way to do that?
> 
> 
>              May I ask what is your use case for that? If you route
>         interrupts to
>              the guest, Xen will not receive vital interrupt such as the
>         timer,
>              UART, SMMU interrupts, maintenance interrupt....
> 
>         I only have one guest domain. So, I'm trying to make Xen
>         transparent to avoid any extra overhead caused by trapping
>         interrupts.
> 
> 
>     Do you include Dom0 in your "one guest domain"? If so, may I ask
>     what is your end goal? Why not booting the OS on baremetal?
> 
> Yes, I only have the Dom0 domain and no DomU. My end goal is to make Xen 
> transparent to the interrupts but have control over the memory 
> management and be able to create another domain (like a driver domain) 
> on-demand whenever I need it.

Then you really don't want bypass Xen for the interrupts... If you do 
that, you will lose the ability to communicate between physical CPUs in 
the hypervisor.

> 
>         But I still need Xen for my own hypercalls. I don't need the
>         timer cause I pinned and don't need any vcpu scheduler.
> 
> 
>     Well, Xen still needs interrupts for other things like UART and
>     SMMU. It also needs interrupts to IPI other pCPU such as for
>     softirq, unblocking another vCPU (waiting on an event for
>     instance)... I don't think you can discard interrupts that easily in
>     Xen without some cooperation with the guest.
> 
>     Let's imagine Xen IPIs another pCPU. If interrupts are routed to
>     your guest, this guest will receive the IPIs and will not understand
>     what to do.
> 
> My end goal is that all the IPIs are handled in the guest like when it 
> is on baremetal.

You keep saying interrupts should be directly routed to the guest but 
you don't explain why you want that. As I said in my previous e-mail,
trying to remove interrupts from the hypervisor is more a pain
compare to the benefits you will gain. You will be better at
improving the latency when receiving interrupt (AFAIK this is
already be good).

> 
> 
> 
>         Based on my understanding, I can only disable the interrupts on
>         ARM all together using the HCR_EL2 register and we can't pick
>         one interrupt to not trap, right?
> 
> 
>     Depends on your interrupts controller. On GICv4, you will be able to
>     directly injected some LPIs (i.e MSI).
> 
> 
> 
> 
> 
>                  I know that HCR_IMO bit in the HCR_EL2 register is
>         supposed to
>                  be for routing the interrupts to the guest (Routing to EL1
>                  instead of EL2).
>                  link to the datasheet:
>         http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500d/CIHJHAAG.html
>         <http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500d/CIHJHAAG.html>
>                 
>         <http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500d/CIHJHAAG.html
>         <http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500d/CIHJHAAG.html>>
> 
>                  So, I have tried doing the following in
>                  the leave_hypervisor_tail. I run a simple hypercall and
>         do the
>                  following lines before return (which is I guess the
>         last point
>                  of exit to the guest from hypervisor):
>                  ---------------------
>                  /current->arch.hcr_el2 &= ~HCR_IMO;/
>                  /WRITE_SYSREG(current->arch.hcr_el2, HCR_EL2);/
>                  /isb();/
>                  /----------------------/
>                  /
>                  /
>                  /It looks like to be doing it right for all
>                  thevcpus butgets stuck after return from
>         leave_hypervisor_tail
>                  for the lastvcpu.
> 
> 
>              What do you mean by stuck? Do you see any logs?
> 
>         It's hung with no log.
> 
> 
>              HCR_EL2.IMO unset means the interrupt will get signaled to
>         EL1. It
>              does not affect how interrupt will get read (e.g IAR).
> 
>              Which interrupt controller are you using?
> 
>         I'm using a GICv2.
> 
>              In case of GICv2, Xen is re-mapping GICC to GICV. So when
>         the guest
>              is reading IAR, it will read the interrupts from the LRs.
>         Not the
>              physical interface.
> 
>            So, in the case of GICv2, we can't route them cause Xen is
>         the one that is updating the LRs and guest is reading from the
>         LRs, am I right?
> 
> 
>     If you want to route *all* the interrupts, you can map GICC and not
>     GICV to your guest. So when the guest will read IAR, it will read
>     the physical interrupts.
> 
> 
> 
>              In case of GICv3, HCR_EL2.IMO will also control the access.
>         So you
>              should be fine here.
> 
>              However, in both case you will at least need to rework the way
>              software generated interrupts are sent to the guest. At the
>         moment,
>              they are written in the LRs.
> 
>         Is it possible to not trap on the ICDSGIR (SGI register)?
> 
> 
>     SGIs register are already trapped by Xen. They are emulated by
>     writing the corresponding interrupt to the LRs.
> 
> How can make it available to the guest and not trap in Xen? Is it doable?

I am afraid you are on your own here. Xen requires interrupts for itself 
(e.g scheduling, timer, event channel...). If you want more information 
then I invite you to read my previous e-mail again.

> 
>     However, SGIs are not the only interrupt generated by the hypervisor
>     directly. There are also the event channel (a PPI) or any device
>     emulated by the hypervisor (e.g PL011).
> 
>     Trying to remove interrupts from the hypervisor is more a pain
>     compare to the benefits you will gain. You will be better at
>     improving the latency when receiving interrupt (AFAIK this is
>     already be good).
> 
> I changed the value of the HCR_EL2 register after doing a hypercall in 
> "leave_hypervisor_tail" and now debugging. I realize that "guest_sync" 
> in entry.S doesn't return back to the guest (I put a print after my 
> hypercall in the guest and doesn't show up). I see that 
> "return_from_trap" gets called. I suppose that eret should take me back 
> to the hvc call I made in the guest. I checked the value of the LR 
> register (link register that store the return address) in 
> "return_from_trap" and there is no change.
> Do you know why changing the value of the HCR_EL2 register should affect 
> returning back to the guest?  Also, without changing the register value 
> I get back to the guest from Xen normally.

While eret means returning in the guest, you may receive an exception 
(e.g interrupt) in the guest diverting from the "normal" path. Hence why 
you don't see the message after the "hvc". It is likely possible you get 
stuck in the exception handler.

Cheers,

-- 
Julien Grall

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