Re: [Qemu-devel] [PATCH v1 2/2] target-arm: Enable EL2 for the A53s and A57s

From: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Alistair Francis <alistair.francis@xilinx.com>,
	Edgar Iglesias <edgar.iglesias@xilinx.com>,
	qemu-arm <qemu-arm@nongnu.org>,
	QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH v1 2/2] target-arm: Enable EL2 for the A53s and A57s
Date: Tue, 10 May 2016 12:36:37 +0200	[thread overview]
Message-ID: <20160510103637.GH16305@toto> (raw)
In-Reply-To: <CAFEAcA84aHCj2ADGkuxEbdx1qy7scu+nXSTU5K70ZPXR2PL1Ng@mail.gmail.com>

On Tue, May 10, 2016 at 09:09:55AM +0100, Peter Maydell wrote:
> On 10 May 2016 at 01:16, Alistair Francis <alistair.francis@xilinx.com> wrote:
> > It is actually a u-boot problem. I originally just assumed it was a
> > Linux problem, but it happens before u-boot hands off to Linux.
> 
> OK, that makes sense. u-boot tends to be a bit lower level and less
> hardware-agnostic. I just wanted to check it wasn't caused by some
> problem in QEMU's EL3 support we could easily fix.
> 
> > It appears that u-boot won't work at all with EL3 enabled but EL2
> > disabled. It always moves to EL2 before moving to EL1 and there is no
> > code prepared to handle going from EL3 directly to EL1.
> >
> > Just for the record, I'm specifically talking about what happens in
> > the do_nonsec_virt_switch() function.
> >
> > It looks like there are three options:
> >  1. Add support to u-boot to drop from EL3 to EL1 (I'm assuming this
> > is possible, as not all implementations have EL2)
> >  2. Just wait until QEMU adds EL2 support
> >  3. Allow a QEMU command line option to start in EL1 instead of EL3

Alistair,

Our usual ZynqMP boot flow is

ATF at EL3
ATF hands over to u-boot at EL2
u-boot hands over to the Linux kernel at EL2

Our ATF has a mode where it can be instructed to start u-boot in
EL1, bypassing EL2. That would avoid this issue. You don't need to
recompile ATF. We can discuss the details off-line.

> I would be OK with any of these three from a QEMU perspective.
> Fixing u-boot is probably conceptually the nicest but I've never
> looked at u-boot internals so it could be simple or painful...
> 
> Edgar, do you have the list of what we're still missing before we
> can turn on EL2?

We're missing the Data Abort ISS:s, we're missing modelling of a few registers
(like HSTR_EL2). Some regs are quite incomplete like HCR but I think we can
live with that. GICv2 virtualization is missing. We are also missing a few
AT_ modes. AArch32 support is probably lacking alot of more stuff.

IIRC, last time this came up we talked about enabling EL2 after ISS and
GICv2 virtualization support gets merged + a few odd regs. Enough to allow
Xen and KVM to work under emulation.

Cheers,
Edgar