Re: [PATCH 1/2] spapr: Disable legacy virtio devices for pseries-5.0 and later

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Feb 07, 2020 at 03:30:54PM +1100, David Gibson wrote:
> PAPR specifies a kind of odd, paravirtualized PCI bus, which looks to
> the guess mostly like classic PCI, even if some of the individual
> devices on the bus are PCI Express.  One consequence of that is that
> virtio-pci devices still default to being in transitional mode, though
> legacy mode is now disabled by default on current q35 x86 machine
> types.
> 
> Legacy mode virtio devices aren't really necessary any more, and are
> causing some problems for future changes.  Therefore, for the
> pseries-5.0 machine type (and onwards), switch to modern-only
> virtio-pci devices by default.

It's worth noting in the commit log that this disables support
for guests older than Linux 4.0.

> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/ppc/spapr.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index c9b2e0a5e0..216d3b34dc 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -65,6 +65,7 @@
>  
>  #include "hw/pci/pci.h"
>  #include "hw/scsi/scsi.h"
> +#include "hw/virtio/virtio-pci.h"
>  #include "hw/virtio/virtio-scsi.h"
>  #include "hw/virtio/vhost-scsi-common.h"
>  
> @@ -4512,7 +4513,14 @@ static const TypeInfo spapr_machine_info = {
>   */
>  static void spapr_machine_5_0_class_options(MachineClass *mc)
>  {
> -    /* Defaults for the latest behaviour inherited from the base class */
> +    /* Most defaults for the latest behaviour are inherited from the
> +     * base class, but we need to override the (non ppc specific)
> +     * default behaviour for virtio */
> +    static GlobalProperty compat[] = {
> +        { TYPE_VIRTIO_PCI, "disable-legacy", "on", },
> +    };
> +
> +    compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
>  }
>  
>  DEFINE_SPAPR_MACHINE(5_0, "5.0", true);

So this sets the defaults, right?

Problem is we'll then need to remember to carry this in the latest
call. If we forget we get a mess.

How about adding a call to e.g. spapr_machine_latest_class_options
in DEFINE_SPAPR_MACHINE?
Then spapr_machine_latest_class_options can set the per-machine
defaults.

I send a patch for this:
	[PATCH] ppc: function to setup latest class options
feel free to reuse.


> @@ -4523,11 +4531,15 @@ DEFINE_SPAPR_MACHINE(5_0, "5.0", true);
>  static void spapr_machine_4_2_class_options(MachineClass *mc)
>  {
>      SpaprMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
> +    static GlobalProperty compat[] = {
> +        { TYPE_VIRTIO_PCI, "disable-legacy", "auto" },
> +    };
>  
>      spapr_machine_5_0_class_options(mc);
>      compat_props_add(mc->compat_props, hw_compat_4_2, hw_compat_4_2_len);
>      smc->default_caps.caps[SPAPR_CAP_CCF_ASSIST] = SPAPR_CAP_OFF;
>      smc->default_caps.caps[SPAPR_CAP_FWNMI_MCE] = SPAPR_CAP_OFF;
> +    compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
>  }
>  
>  DEFINE_SPAPR_MACHINE(4_2, "4.2", false);
> -- 
> 2.24.1