linux-kernel - Re: [PATCH v6 13/18] KVM: arm64: Support SDEI_EVENT_{COMPLETE,COMPLETE_AND

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Message-ID: <Ym+KdY2y2vzMeDOL@google.com>
Date:   Mon, 2 May 2022 07:38:29 +0000
From:   Oliver Upton <oupton@...gle.com>
To:     Gavin Shan <gshan@...hat.com>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        eauger@...hat.com, Jonathan.Cameron@...wei.com,
        vkuznets@...hat.com, will@...nel.org, shannon.zhaosl@...il.com,
        james.morse@....com, mark.rutland@....com, maz@...nel.org,
        pbonzini@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v6 13/18] KVM: arm64: Support
 SDEI_EVENT_{COMPLETE,COMPLETE_AND_RESUME} hypercall

On Mon, May 02, 2022 at 02:19:30PM +0800, Gavin Shan wrote:
> Hi Oliver,
> 
> 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.

--
Thanks,
Oliver