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From:   Marc Zyngier <maz@kernel.org>
Date:   Tue, 06 Jul 2021 11:36:17 +0100
Message-ID: <87czrv91b2.wl-maz@kernel.org>
rom:    Marc Zyngier <maz@kernel.org>
To:     Sumit Garg <sumit.garg@linaro.org>
Cc:     Jens Wiklander <jens.wiklander@linaro.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        OP-TEE TrustedFirmware <op-tee@lists.trustedfirmware.org>,
        Devicetree List <devicetree@vger.kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        Jerome Forissier <jerome@forissier.org>,
        Etienne Carriere <etienne.carriere@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH v2 0/7] Asynchronous notifications from secure world
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On Tue, 06 Jul 2021 08:25:26 +0100,
Sumit Garg <sumit.garg@linaro.org> wrote:
> 
> On Thu, 17 Jun 2021 at 11:40, Jens Wiklander <jens.wiklander@linaro.org> wrote:
> >
> > Hi Sumit,
> >
> > On Thu, Jun 17, 2021 at 6:33 AM Sumit Garg <sumit.garg@linaro.org> wrote:
> > >
> > > Hi Jens,
> > >
> > > On Wed, 16 Jun 2021 at 16:07, Jens Wiklander <jens.wiklander@linaro.org> wrote:
> > > >
> > > > Hi all,
> > > >
> > > > This adds support for asynchronous notifications from OP-TEE in secure
> > > > world to the OP-TEE driver. This allows a design with a top half and bottom
> > > > half type of driver where the top half runs in secure interrupt context and
> > > > a notifications tells normal world to schedule a yielding call to do the
> > > > bottom half processing.
> > > >
> > > > An interrupt is used to notify the driver that there are asynchronous
> > > > notifications pending.
> > > >
> > >
> > > It looks like a nice feature. I would like to get hands on with this.
> > > Can I test this feature on Qemu?
> >
> > Absolutely, you can get this into the normal OP-TEE development repo setup with:
> > repo init -u https://github.com/OP-TEE/manifest.git -m default.xml
> > repo sync
> > Update optee_os with
> > https://github.com/jenswi-linaro/optee_os/tree/async_notif_v2
> > Update linux with https://github.com/jenswi-linaro/linux-1/tree/async_notif_v2
> > cd build
> > make all -j...
> > make run-only
> >
> > If you type anything at the secure console you'll notice how it
> > changes behaviour once the Linux kernel has booted.
> >
> 
> Thanks for sharing instructions as I now got some time to test and
> deep dive into this feature. It looks like a pretty useful feature to
> realize interrupt support in the secure world in its true sense. This
> feature works for me as per your instructions.
> 
> I could recognise it's requirement from the time while I was playing
> with secure timer interrupt support for OP-TEE RNG driver on
> Developerbox. In that case I had to strip down the secure interrupt
> handler to a minimum that would just collect entropy and dump into the
> secure buffer. But with asynchronous notifications support, I could
> add more functionality like entropy health tests in the bottom half
> instead of doing those health tests while retrieving entropy from the
> secure world.
> 
> Given that, have you explored the possibility to leverage SGI rather
> than a platform specific SPI for notifying the normal world? If it's
> possible to leverage Architecture specific SGI for this purpose then I

What does "Architecture specific SGI" mean?

> think this feature will come automatically enabled for every platform
> without the need to reserve a platform specific SPI.

That old chestnut again...

- How do you discover that the secure side has graced you with a
  Group-1 SGI (no, you can't use one of the first 8)? for both DT and
  ACPI?

- How do you find which CPUs are targeted by this SGI? All? One? A
  subset? What is the expected behaviour with CPU hotplug? How can the
  NS side (Linux) can inform the secure side about the CPUs it wants
  to use?

- Is there any case where you would instead need a level interrupt
  (which a SGI cannot provide)?

In general, cross world SGIs are a really bad idea. Yes, some people
like them. I still think they are misguided, and I don't intend to
provide a generic request interface for this.

	M.

-- 
Without deviation from the norm, progress is not possible.