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

From: Alexander Graf
Subject: Re: [Qemu-devel] [RFC 03/19] sysbus: Set user_creatable=false by default on TYPE_SYS_BUS_DEVICE
Date: Mon, 3 Apr 2017 22:15:44 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

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 <address@hidden> wrote:
commit 33cd52b5d7b9adfd009e95f07e6c64dd88ae2a31 unset
cannot_instantiate_with_device_add_yet in TYPE_SYSBUS, making
all kinds of untested devices available to -device and

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:

* 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.


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