Re: [Qemu-devel] [PATCH RESEND v2 1/3] pc: Set hw_version on all machine classes

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 30, 2015 at 05:36:07PM -0200, Eduardo Habkost wrote:
> In 2012, QEMU had a bug where it exposed QEMU version information to the
> guest, meaning a QEMU upgrade would expose different hardware to the
> guest OS even if the same machine-type is being used.
> 
> The bug was fixed by commit 93bfef4c6e4b23caea9d51e1099d06433d8835a4, on
> all machines up to pc-1.0. But we kept introducing the same bug on all
> newer machines since then. That means we are breaking guest ABI every
> time QEMU was upgraded.
> 
> Fix this by setting the hw_version on all PC machines, making sure the
> hardware won't change when upgrading QEMU.
> 
> Note that QEMU_VERSION was "1.0" in QEMU 1.0, but starting on QEMU
> 1.1.0, it started following the "x.y.0" pattern. We have to follow it,
> to make sure we use the right QEMU_VERSION string from each QEMU
> release.
> 
> The 2.5 machine classes could have hw_version unset, because the default
> value for qemu_get_version() is QEMU_VERSION. But I decided to set it
> explicitly to QEMU_VERSION so we don't forget to update it to "2.5.0"
> after we release 2.5.0 and create a 2.6 machine class.
> 
> Reported-by: Laszlo Ersek <lersek@redhat.com>
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

Ouch.

I really don't want even more churn with each version.

Can't we use the name supplied to DEFINE_PC_MACHINE at least for future
machine types?

Or maybe we should stop exposing the version to guests - does it really
have any value given it has been so unreliable historically?

How about:

---

diff --git a/util/osdep.c b/util/osdep.c
index 0092bb6..4dc635d 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -52,7 +52,7 @@ extern int madvise(caddr_t, size_t, int);
 
 static bool fips_enabled = false;
 
-static const char *qemu_version = QEMU_VERSION;
+static const char *qemu_version = "QEMU";
 
 int socket_set_cork(int fd, int v)
 {