Re: [Qemu-devel] [RFC 03/19] sysbus: Set user_creatable=false by default on TYPE_SYS_BUS_DEVICE

[Peter Maydell <peter.maydell@linaro.org>]

On 1 April 2017 at 01:46, Eduardo Habkost <ehabkost@redhat.com> wrote:
> commit 33cd52b5d7b9adfd009e95f07e6c64dd88ae2a31 unset
> cannot_instantiate_with_device_add_yet in TYPE_SYSBUS, making
> all kinds of untested devices available to -device and
> device_add.
>
> The problem with that is: setting has_dynamic_sysbus on a
> machine-type lets it accept all the 288 sysbus device types we
> have in QEMU, and most of them were never meant to be used with
> -device. That's a lot of untested code.
>
> Fortunately today we have just a few has_dynamic_sysbus=1
> machines: virt, pc-q35-*, ppce500, and spapr.
>
> virt, ppce500, and spapr have extra checks to ensure just a few
> device types can be instantiated:
>
> * virt supports only TYPE_VFIO_CALXEDA_XGMAC, TYPE_VFIO_AMD_XGBE.
> * ppce500 supports only TYPE_ETSEC_COMMON.
> * spapr supports only TYPE_SPAPR_PCI_HOST_BRIDGE.
>
> q35 has no code to block unsupported sysbus devices, however, and
> accepts all device types. Fortunately, only the following 20
> device types are compiled into the qemu-system-x86_64 and
> qemu-system-i386 binaries:
>
> * allwinner-ahci
> * amd-iommu
> * cfi.pflash01
> * esp
> * fw_cfg_io
> * fw_cfg_mem
> * generic-sdhci
> * hpet
> * intel-iommu
> * ioapic
> * isabus-bridge
> * kvmclock
> * kvm-ioapic
> * kvmvapic
> * SUNW,fdtwo
> * sysbus-ahci
> * sysbus-fdc
> * sysbus-ohci
> * unimplemented-device
> * virtio-mmio
>
> Instead of requiring each machine-type with has_dynamic_sysbus=1
> to implement its own mechanism to block unsupported devices, we
> can use the user_creatable flag to ensure we won't let the user
> plug anything that will never work.

How does this work? Which devices can be dynamically
plugged is machine dependent. You can't dynamically-plug
an intel-iommu on the ARM virt board, and you can't
dynamically-plug the vfio-calxeda-xgmac on the spapr
board, and so on. So I don't see how we can just have
a flag on the device itself that controls whether
it can be dynamically plugged.

So I'm definitely coming around to the opinion that
it's just a bug in the q35 board that it doesn't have
any device whitelisting, and we should fix that.

thanks
-- PMM