Re: [EXT] Re: QEMU+KVM on RISC-V + Hypervisor Extension

[Ralf Ramsauer]

On 21/03/2022 21:47, Palmer Dabbelt wrote:
> On Sun, 20 Mar 2022 22:43:07 PDT (-0700), alistair23@gmail.com wrote:
> > On Thu, Mar 17, 2022 at 7:46 PM Peter Maydell 
> > <peter.maydell@linaro.org> wrote:
> > > On Wed, 16 Mar 2022 at 22:23, Alistair Francis <alistair23@gmail.com> 
> > > wrote:
> > > > Hmm... This seems like a bug. We shouldn't allow the user to specify a
> > > > `-bios` option if using KVM. Would you mind preparing a patch to catch
> > > > this?
> > >
> > > You don't want to allow the possibility of a bios blob that expects
> > > to run in S-mode, the way arm virt can run an EL1 UEFI BIOS ?
> >
> > Interesting. We could still allow that by using -device loader though.
> >
> > If we load something in S mode it really is a kernel and not firmware,
> > so I think the -bios argument is a little misleading. We could make
> > -bios and -enable-kvm a warning instead, to discourage users from
> > providing OpenSBI, but still allow them to pass something.
>
> We could handle this in OpenSBI: have it just detect that it's been 
> launched in S-mode and just jump to the payload.
>
> > To me not allowing it seems reasonable, and we can always re-allow it
> > in the future if there is a good use case.
>
> That also seems fine to me.  There'd be a lot of work involved in doing 
> anything useful with S-mode firmware, given that things aren't really 
> designed to work that way.  Certainly a lot more work than re-enabling 
> this sort of flag, and while it might be a bit of a headache coupling 
> that to a QEMU source-level change my guess is that there'd need to be a 
> bunch of work done on new HW interfaces to make this useful so there'd 
> be QEMU changes to emulate those either way.
>
> Having at least a warning seems prudent, as most users are going to end 
> up with a hang here and a silent hang is a headache for everyone.

Okay, my analysis was wrong, -bios is in fact already ignored when being 
used in combination with -enable-kvm on the virt machine model. See:

https://git.qemu.org/?p=qemu.git;a=blob;f=hw/riscv/virt.c;h=da50cbed43ec54777992d40dbf158ec63fccef03;hb=HEAD#l1314

However, the bios will be silently ignored and dropped, which led me to 
the mistake. I'll provide a patch that gives a warning to the user.

  Ralf

> > Alistair
> >
> > > thanks
> > > -- PMM