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Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virti

From: Anthony Liguori
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device
Date: Wed, 21 Mar 2012 11:26:50 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.2) Gecko/20120216 Thunderbird/10.0.2

On 03/21/2012 11:11 AM, Michael S. Tsirkin wrote:
On Wed, Mar 21, 2012 at 10:14:35AM -0500, Anthony Liguori wrote:
On 03/21/2012 10:10 AM, Michael S. Tsirkin wrote:
On Wed, Mar 21, 2012 at 09:42:41AM -0500, Anthony Liguori wrote:
On 03/21/2012 08:08 AM, Michael S. Tsirkin wrote:
On Wed, Mar 21, 2012 at 11:26:15AM +0000, Stefan Hajnoczi wrote:
On Tue, Mar 20, 2012 at 09:19:47PM +1100, David Gibson wrote:
Looking at hw/pc_piix.c there are QEMUMachine types for each QEMU
release.  Legacy machine types (e.g. pc_machine_v0_14) have a
.compat_props array that can override qdev properties.

Perhaps Michael Tsirkin or someone else can comment on how to wire up
hw/virtio-pci.c so that the class code can be overridden.


afaik we already let users over-write it for some other pci devices,
look there for examples.

 From hw/pc_piix.c:

     .name = "pc-0.10",
     .desc = "Standard PC, qemu 0.10",
     .init = pc_init_pci_no_kvmclock,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
             .driver   = "virtio-blk-pci",
             .property = "class",
             .value    = stringify(PCI_CLASS_STORAGE_OTHER),

And from the earlier part of the thread, yes, it's imperative that
we do not change anything in the PCI configuration space for older
pc versions regardless of whether it may or may not work.

Certain guests (like Windows) use a complex fingerprinting algorithm
to determine when hardware changes.  It can be hard to detect in
simple testing because it's based on a threshold.


Anthony Liguori

Which reminds me - qemu sticks the release version in
guest visible places like CPU version.
This is wrong and causes windows guests to print messages
about driver updates when you switch.
We should find all these places and stop doing this.

We could probably get away with doing a query/replace of
QEMU_VERSION with qemu_get_version(), make version a static variable
that defaults to QEMU_VERSION, and then provide a way for machines
to override it.

Then pc-0.10 could report a version of 0.10.


Anthony Liguori

Frankly I don't see value in making it visible to the user,
at all. We are just triggering windows reactivations
without any user benefit. Why not return a fixed value there
to avoid that?

I don't see a problem making it fixed for 1.1, but for 1.0 and older, we should expose what we were supposed to expose.

We need to fix the bug first, then we can change the behavior.


Anthony Liguori

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