Re: [PATCH v6 13/18] KVM: arm64: Support SDEI_EVENT_{COMPLETE,COMPLETE_AND_RESUME} hypercall

[Gavin Shan]

Hi Oliver,

On 5/2/22 3:38 PM, Oliver Upton wrote:
> On Mon, May 02, 2022 at 02:19:30PM +0800, Gavin Shan wrote:
> > On 5/1/22 2:50 PM, Oliver Upton wrote:
> > > On Sun, Apr 03, 2022 at 11:39:06PM +0800, Gavin Shan wrote:
> > > > This supports SDEI_EVENT_{COMPLETE, COMPLETE_AND_RESUME} hypercall.
> > > > They are used by guest to notify the completion of event in its
> > > > handler. The previously interrupted or preempted context is restored
> > > > like below.
> > > >
> > > >      * x0 - x17, PC and PState are restored to what values we had in
> > > >        the interrupted or preempted context.
> > > >
> > > >      * If it's SDEI_EVENT_COMPLETE_AND_RESUME hypercall, IRQ exception
> > > >        is injected.
> > >
> > > I don't think that's how COMPLETE_AND_RESUME works. The caller specifies an
> > > address at which it would like to begin execution within the client
> > > exception level.
> > >
> > > SDEI spec suggests this behaves like a synchronous exception. DEN 0054C
> > > 5.2.2 'Event Resume Context' speaks more about how it is supposed to
> > > work.
> >
> > It's actually the linux convention. If the event handler, which was
> > specified in previous hypercall to EVENT_REGISTER, returns success,
> > the (linux) client calls into COMPLETE_AND_RESUME and the resume
> > address is specified with FIQ vector offset. More details can be
> > found from arch/arm64/kernel::sdei.c::do_sdei_event().
>
> Right -- but look at what its doing. It returns the address at which it
> wants to resume execution.
>
> arch/arm64/kernel.entry.S::__sdei_asm_handler winds up passing this as
> an argument to COMPLETE_AND_RESUME. Also, what would happen if we run
> something that isn't Linux inside of KVM? This is why I suggested
> implementing COMPLETE_AND_RESUME in line with the specification, not
> based on what the kernel is presently doing.

Indeed. The address for the resumed execution is passed by x1 when
COMPLETE_AND_RESUME is called. I will figure this out in next revision.
I don't think we can have the assumption that the guest is linux.

Thanks again for your review and comments :)

Thanks,
Gavin