Re: [Qemu-devel] [RFC v4 4/5] hw/arm/virt: Add 2.10 machine type

[Auger Eric <eric.auger@redhat.com>]

Hi Peter,

On 30/05/2017 18:04, Peter Maydell wrote:
> On 13 May 2017 at 18:43, Eric Auger <eric.auger@redhat.com> wrote:
>> The new machine type allows smmuv3 instantiation. A new option
>> is introduced to turn the feature on/off (off by default).
> 
> Should we go for default-on, or would that break guests?
> For other things added to the virt board I think the
> approach we've taken has been:
>  * if this is just an extra device, simply provide it in the
>    new virt-n.nn machine by default
>  * if this is something that changes how the machine behaves
>    even for code that doesn't care about that feature (eg
>    EL2, EL3, since they change what mode you start in on boot)
>    then default it to off
>  * if the feature only works with TCG and not KVM then
>    maybe default it to off

OK thanks for the guidelines. I put it off by default since it can
largely degrade the performance in some use cases. Also instantiating
the smmu can also induce some option changes at other levels: for
instance virtio-pci devices may be used along with iommu_platform option
for the guest to use dma ops.

Thanks

Eric
> 
> thanks
> -- PMM
>