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

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Apr 03, 2017 at 10:15:44PM +0200, Alexander Graf wrote:
> 
> 
> On 03.04.17 22:10, Eduardo Habkost wrote:
> > On Mon, Apr 03, 2017 at 08:49:16PM +0100, Peter Maydell wrote:
> > > 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.
> > 
> > OK, let's assume q35 must implement a whitelist:
> > 
> > To build that whitelist, we need to be able to know what should
> > be in the whitelist, or not. And nobody knew for sure what was
> > user-creatable in q35 by accident, and what was really supposed
> > to be user-creatable in q35. See the "q35 and sysbus devices"
> > thread I started ~2 weeks ago.
> > 
> > Building a q35 whitelist will be much easier if make
> > sys-bus-devices non-user-creatable by default.
> 
> So why are they user creatable in the first place?
> 
> We used to have boards that were dynamic sysbus aware (ppce500, virt) that
> allowed dynamic creation and every other board did not. I don't remember the
> exact mechanism behind it though.
> 
> When did that behavior change? It sounds like a regression somewhere.

See patch description:

> > > > commit 33cd52b5d7b9adfd009e95f07e6c64dd88ae2a31 unset
> > > > cannot_instantiate_with_device_add_yet in TYPE_SYSBUS,

Note that the commit above is not a regression[1] (because q35
didn't have has_dynamic_sysbus=1 yet), but having sysbus devices
have cannot_instantiate_with_device_add_yet=false/user_creatable=true
by default makes it harder to build the whitelist for q35 (or
other machines that will have has_dynamic_sysbus=1 in the
future).

[1] Well, it was a regression on the "info qdm" HMP command
    output, which is a minor bug. But it's still a bug we still
    want to fix anyway.

-- 
Eduardo