Re: [PATCH v6 07/11] hvf: Add Apple Silicon support

From: Peter Maydell <peter.maydell@linaro.org>
To: Alexander Graf <agraf@csgraf.de>
Cc: Eduardo Habkost <ehabkost@redhat.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Cameron Esfahani <dirty@apple.com>,
	Roman Bolshakov <r.bolshakov@yadro.com>,
	qemu-arm <qemu-arm@nongnu.org>, Frank Yang <lfy@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Peter Collingbourne <pcc@google.com>
Subject: Re: [PATCH v6 07/11] hvf: Add Apple Silicon support
Date: Thu, 11 Feb 2021 13:16:00 +0000	[thread overview]
Message-ID: <CAFEAcA9bmV4jUubwqrQXzPXMFfNCsyw7XD34dLW6HBU7_uQvRQ@mail.gmail.com> (raw)
In-Reply-To: <37018444-82a8-96c0-b5ce-da056646a1b8@csgraf.de>

On Thu, 11 Feb 2021 at 13:06, Alexander Graf <agraf@csgraf.de> wrote:
>
>
> On 10.02.21 23:39, Peter Maydell wrote:
> > On Wed, 10 Feb 2021 at 22:21, Alexander Graf <agraf@csgraf.de> wrote:
> >>
> >> On 28.01.21 16:52, Peter Maydell wrote:
> >>> On Wed, 20 Jan 2021 at 22:44, Alexander Graf <agraf@csgraf.de> wrote:
> >>>> +            break;
> >>>> +        case EC_AA64_SMC:
> >>>> +            cpu_synchronize_state(cpu);
> >>>> +            if (arm_is_psci_call(arm_cpu, EXCP_SMC)) {
> >>>> +                arm_handle_psci_call(arm_cpu);
> >>> Have you checked that all the PSCI code really can cope
> >>> with being called from a non-TCG accelerator? (As an example
> >>> the CPU_SUSPEND implementation calls the TCG wfi helper...)
> >>
> >> I have not explicitly tried it, but I don't see why the TCG
> >> implementation of wfi should in principle break with hvf.
> > Because the TCG implementation of wfi is "set some state fields
> > and then longjump out to the TCG exec_cpu code-execution loop",
> > and hvf doesn't use that loop.
>
>
> I can confirm that it breaks, but are you really sure about the longjmp
> not working?
>
> What would you prefer instead? Duplicate the PSCI implementation for HVF?

I would prefer that you worked through the details. In other
words, mostly my concerns with this series are that it feels
like it has a lot of quick-hack "this makes the guests I tested
boot" stuff in it. Examples include this PSCI handling, the
WFI/timer interrupt bits, the way the GIC is done, and the
"let's ignore bogus SMC calls" below.

> >>> This should inject an UNDEF exception into the guest. (Compare
> >>> the pre_smc helper in target/arm/op_helper.c for TCG.)
> >>
> >> That would break Windows, which is one of the main use cases for hvf
> >> support in QEMU.
> > Why is Windows making bogus SMC calls ?
>
>
> Let me have a quick at my crystal ball ... mmmmmmhhhh ... it's a bit
> blurry unfortunately.
>
> I really don't think I'm the right person to answer that question :).
> But the Windows loader does invoke weird SMC calls on boot:

Does it boot under TCG ? Under KVM ?

If there's an SMC API that we ought to be implementing but aren't,
then we should implement it consistently. If the guest is doing
something wrong, we shouldn't put in fudges to work around that.
Once that kind of hack gets into the codebase it is practically
impossible to ever get rid of it.

thanks
-- PMM