Re: [PATCH 1/1] ppc/pnv.c: add a friendly warning when accel=kvm is used

[Cédric Le Goater]

On 11/26/21 02:11, David Gibson wrote:
> On Thu, Nov 25, 2021 at 07:42:02PM -0300, Daniel Henrique Barboza wrote:
> > If one tries to use -machine powernv9,accel=kvm in a Power9 host, a
> > cryptic error will be shown:
> >
> > qemu-system-ppc64: Register sync failed... If you're using kvm-hv.ko, only "-cpu 
> > host" is possible
> > qemu-system-ppc64: kvm_init_vcpu: kvm_arch_init_vcpu failed (0): Invalid 
> > argument
> >
> > Appending '-cpu host' will throw another error:
> >
> > qemu-system-ppc64: invalid chip model 'host' for powernv9 machine
> >
> > The root cause is that in IBM PowerPC we have different specs for the bare-metal
> > and the guests. The bare-metal follows OPAL, the guests follow PAPR. The kernel
> > KVM modules presented in the ppc kernels implements PAPR. This means that we
> > can't use KVM accel when using the powernv machine, which is the emulation of
> > the bare-metal host.
> >
> > All that said, let's give a more informative error in this case.
> >
> > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > ---
> >   hw/ppc/pnv.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/hw/ppc/pnv.c b/hw/ppc/pnv.c
> > index 71e45515f1..e5b87e8730 100644
> > --- a/hw/ppc/pnv.c
> > +++ b/hw/ppc/pnv.c
> > @@ -742,6 +742,11 @@ static void pnv_init(MachineState *machine)
> >       DriveInfo *pnor = drive_get(IF_MTD, 0, 0);
> >       DeviceState *dev;
> >   
> > +    if (kvm_enabled()) {
> > +        error_report("The powernv machine does not work with KVM 
> > acceleration");
> > +        exit(EXIT_FAILURE);
> > +    }
>
>
> Hmm.. my only concern here is that powernv could, at least
> theoretically, work with KVM PR.  I don't think it does right now,
> though.

At the same time, it is nice to not let the user think that it could work
in its current state. Don't you think so ?

C.

> > +
> >       /* allocate RAM */
> >       if (machine->ram_size < mc->default_ram_size) {
> >           char *sz = size_to_str(mc->default_ram_size);