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Re: [PATCH] arm: Don't remove EL3 exposure for SMC conduit

From: Alex Bennée
Subject: Re: [PATCH] arm: Don't remove EL3 exposure for SMC conduit
Date: Mon, 15 Nov 2021 12:08:29 +0000
User-agent: mu4e 1.7.5; emacs 28.0.60

Alexander Graf <agraf@csgraf.de> writes:

> On 15.11.21 11:46, Peter Maydell wrote:
>> On Sun, 14 Nov 2021 at 17:41, Alexander Graf <agraf@csgraf.de> wrote:
>>>> Am 14.11.2021 um 18:20 schrieb Peter Maydell <peter.maydell@linaro.org>:
>>>> This is tricky, because we use the cpu->isar values to determine whether
>>>> we should be emulating things. So this change means we now create an
>>>> inconsistent CPU which in some ways claims to have EL3 (the ISAR ID
>>>> bits say so) and in some ways does not (the ARM_FEATURE_EL3 flag is
>>>> unset), and depending on which of the two "do we have EL3?" methods
>>>> any bit of the TCG code is using will give different results...
>>> Do you think it would be sufficient to go through all readers of
>>> the isar bits and guard them behind an ARM_FEATURE_EL3 check in
>>> addition? I'll be happy to do so then! :)
>> That would be a big reverse-course on a design choice we made that
>> the preference is to look at the ID registers and phase out the
>> use of ARM_FEATURE bits where possible.
> I'm open to alternatives. As it stands, we're lying to the guest
> because we tell it "SMC is not available" but ask it to call SMC for
> PSCI, which is bad too.

Is testing the ISAR bits actually telling a guest that SMC exists or
just the CPU is capable of handling it? I guess -kernel only is a weird
case because otherwise if EL3 is available some sort of firmware has to
have gotten the CPU into a state a kernel can boot. It doesn't imply
that firmware knows how to do a PSCI call though - surely there is some
firmware configuration/probing mechanism you need to rely on for that?

Alex Bennée

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