Re: [PATCH v3 1/2] spapr: Disable legacy virtio devices for pseries-5.0 and later

[Greg Kurz <groug@kaod.org>]

On Thu,  5 Mar 2020 15:30:08 +1100
David Gibson <david@gibson.dropbear.id.au> 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.
> 
> This does mean we no longer support guest kernels prior to 4.0, unless
> they have modern virtio support backported (which some distro kernels
> like that in RHEL7 do).
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---

Reviewed-by: Greg Kurz <groug@kaod.org>

FWIW, I could test the following:
- allows a RHEL7 guest with pre 4.0 kernel to boot, as mentioned
  in the changelog
- breaks boot of older RHEL 6.10 guests as expected
- allows migration of older machine types to/from QEMU 4.2

Tested-by: Greg Kurz <groug@kaod.org>

>  hw/ppc/spapr.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 2eb0d8f70d..3cfc98ac61 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"
>  
> @@ -4566,8 +4567,20 @@ static const TypeInfo spapr_machine_info = {
>  
>  static void spapr_machine_latest_class_options(MachineClass *mc)
>  {
> +    /*
> +     * 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.  We can't do that from the base
> +     * class since it doesn't have a compat_props.
> +     */
> +    static GlobalProperty compat[] = {
> +        { TYPE_VIRTIO_PCI, "disable-legacy", "on", },
> +    };
> +
>      mc->alias = "pseries";
>      mc->is_default = true;
> +
> +    compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
>  }
>  
>  #define DEFINE_SPAPR_MACHINE(suffix, verstr, latest)                 \
> @@ -4607,6 +4620,9 @@ 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);
> @@ -4614,6 +4630,7 @@ static void spapr_machine_4_2_class_options(MachineClass *mc)
>      smc->default_caps.caps[SPAPR_CAP_FWNMI_MCE] = SPAPR_CAP_OFF;
>      smc->rma_limit = 16 * GiB;
>      mc->nvdimm_supported = false;
> +    compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
>  }
>  
>  DEFINE_SPAPR_MACHINE(4_2, "4.2", false);