Re: [PATCH v4 2/2] hw/i386: Add the ramfb romfile compatatibility

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Jun 27, 2025 at 01:37:55PM +0800, Shaoqin Huang wrote:
> Hi Eric,
> 
> On 6/26/25 4:01 PM, Eric Auger wrote:
> > 
> > 
> > On 6/26/25 4:05 AM, Shaoqin Huang wrote:
> > > Hi Eric,
> > > 
> > > On 6/23/25 5:20 PM, Eric Auger wrote:
> > > > 
> > > > 
> > > > On 6/17/25 5:05 AM, Shaoqin Huang wrote:
> > > > > Set the "use-legacy-x86-rom" property to false by default, and only set
> > > > > it to true on x86 since only x86 will need it.
> > > > s/compatatibility/compatibility in the title
> > > 
> > > Ok. Will fix it.
> > > 
> > > > > 
> > > > > Signed-off-by: Shaoqin Huang <shahuang@redhat.com>
> > > > > ---
> > > > >    hw/display/ramfb-standalone.c | 2 +-
> > > > >    hw/i386/pc_q35.c              | 3 +++
> > > > >    hw/vfio/pci.c                 | 2 +-
> > > > >    3 files changed, 5 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/hw/display/ramfb-standalone.c b/hw/display/ramfb-
> > > > > standalone.c
> > > > > index af1175bf96..ddbf42f181 100644
> > > > > --- a/hw/display/ramfb-standalone.c
> > > > > +++ b/hw/display/ramfb-standalone.c
> > > > > @@ -63,7 +63,7 @@ static const VMStateDescription ramfb_dev_vmstate = {
> > > > >      static const Property ramfb_properties[] = {
> > > > >        DEFINE_PROP_BOOL("x-migrate", RAMFBStandaloneState, migrate,
> > > > > true),
> > > > > -    DEFINE_PROP_BOOL("use-legacy-x86-rom", RAMFBStandaloneState,
> > > > > use_legacy_x86_rom, true),
> > > > > +    DEFINE_PROP_BOOL("use-legacy-x86-rom", RAMFBStandaloneState,
> > > > > use_legacy_x86_rom, false),
> > > > >    };
> > > > >      static void ramfb_class_initfn(ObjectClass *klass, void *data)
> > > > > diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> > > > > index fd96d0345c..f6d89578d0 100644
> > > > > --- a/hw/i386/pc_q35.c
> > > > > +++ b/hw/i386/pc_q35.c
> > > > > @@ -45,6 +45,7 @@
> > > > >    #include "hw/i386/pc.h"
> > > > >    #include "hw/i386/amd_iommu.h"
> > > > >    #include "hw/i386/intel_iommu.h"
> > > > > +#include "hw/vfio/pci.h"
> > > > >    #include "hw/virtio/virtio-iommu.h"
> > > > >    #include "hw/display/ramfb.h"
> > > > >    #include "hw/ide/pci.h"
> > > > > @@ -67,6 +68,8 @@
> > > > >      static GlobalProperty pc_q35_compat_defaults[] = {
> > > > >        { TYPE_VIRTIO_IOMMU_PCI, "aw-bits", "39" },
> > > > > +    { TYPE_RAMFB_DEVICE, "use-legacy-x86-rom", "true" },
> > > > > +    { TYPE_VFIO_PCI, "use-legacy-x86-rom", "true" },
> > > > this will only keep the legacy behavior along with q35 machine type but
> > > > not on other machines being used for x86. what about pc-i440fx? Doesn't
> > > > it apply to it as well? Are there other machine types also impacted.
> > > 
> > > Ok I will also add it with pc-i440fx. I think only q35 and i440fx are
> > > impacted.
> > > 
> > > > 
> > > > Also what about Daniel's comment in v3:
> > > > https://lore.kernel.org/all/aEak8utPPkHepVfR@redhat.com/
> > > > "For non-x86, historical versioned machine types will need
> > > > likely it set to true, in order to avoid the memory layout
> > > > being changed IIUC."
> > > > 
> > > > Is it actually needed?
> > > 
> > > If those machine types need to set it to true. I think they can set it
> > > after they have this property.
> > nope it does not work like that. In case we really need to take care of
> > this, this must be handled by compats.
> 
> If so. Why don't we still keep the "use-legacy-x86-rom" default to true, and
> only set it to false to those arch which doesn't need it just like my
> original implementation.
> 
> Because I don't really know how other arch's memoery layout was impacted by
> this property set to false. I think keep their original behavior and only
> change it on arm64 is a good idea.
> 
> How do you think about it?

No, the default value of the property shoudl reflect the long
term desired behaviour - in this case 'use-legacy-x86-rom = false'.

We must then reverse this default to 'true' for ALL historical versioned
machine types on ALL architectures, where this device is built, or any
specific machines where we want to keep the historical behaviour going
forward.


With regards,
Daniel
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