Re: [PATCH v2 2/2] hw/riscv: sifive_u: Provide a reliable way for bootloader to detect whether it is running in QEMU

[Alistair Francis]

On Thu, Jul 9, 2020 at 5:50 PM Bin Meng <bmeng.cn@gmail.com> wrote:
>
> Hi Palmer,
>
> On Fri, Jul 10, 2020 at 8:45 AM Palmer Dabbelt <palmerdabbelt@google.com> 
> wrote:
> >
> > On Thu, 09 Jul 2020 15:09:18 PDT (-0700), alistair23@gmail.com wrote:
> > > On Thu, Jul 9, 2020 at 3:07 AM Bin Meng <bmeng.cn@gmail.com> wrote:
> > >>
> > >> From: Bin Meng <bin.meng@windriver.com>
> > >>
> > >> The reset vector codes are subject to change, e.g.: with recent
> > >> fw_dynamic type image support, it breaks oreboot again.
> > >
> > > This is a recurring problem, I have another patch for Oreboot to fix
> > > the latest breakage.
> > >
> > >>
> > >> Add a subregion in the MROM, with the size of machine RAM stored,
> > >> so that we can provide a reliable way for bootloader to detect
> > >> whether it is running in QEMU.
> > >
> > > I don't really like this though. I would prefer that we don't
> > > encourage guest software to behave differently on QEMU. I don't think
> > > other upstream boards do this.
> >
> > I agree.  If you want an explicitly virtual board, use the virt board.  
> > Users
> > of sifive_u are presumably trying to do their best to test against what the
> > hardware does without actually using the hardware.  Otherwise there should 
> > be
> > no reason to use the sifive_u board, as it's just sticking a layer of
> > complexity in the middle of everything.
>
> Understood. Then let's drop this patch.
>
> >
> > > Besides Oreboot setting up the clocks are there any other users of this?
> >
> > IIRC we have a scheme for handling the clock setup in QEMU where we accept
> > pretty much any control write and then just return reads that say the PLLs 
> > have
> > locked.  I'd be in favor of improving the scheme to improve compatibility 
> > with
> > the actual hardware, but adding some way for programs to skip the clocks
> > because they know they're in QEMU seems like the wrong way to go.
> >
>
> Yep, that's my question to Oreboot too.
>
> U-Boot SPL can boot with QEMU and no problem was seen with clock
> settings in PRCI model in QEMU.

I don't think it's an unsolvable problem. There is just little work on
Oreboot to run on QEMU. I can dig into it a bit and see if I can find
a better fix on the Oreboot side.

Alistair

>
> Regards,
> Bin