Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host

[Jamie Lokier <jamie@shareable.org>]

Andrea Arcangeli wrote:
> Hi Jamie,
> 
> thanks for review!
> 
> On Wed, Jun 24, 2009 at 06:29:34PM +0100, Jamie Lokier wrote:
> > So your patch should make no difference to Linux guests.  Did you
> > check Linux behaviour?  Does Windows ignore the vendor id?
> 
> Maybe I wasn't clear enough, this patch only makes a difference for
> KVM. KVM switches the vendor id to Intel when run on vmx host. It
> definitely makes a difference to linux guest, without this patch 32bit
> linux run as guest on top of KVM has sep disabled in /proc/cpuinfo and
> isn't using sysenter (just like windows, hence the skype crash). I
> verified 32bit linux guest has sep enabled with this patch applied
> when run on KVM on top of intel.

That was not at all clear.

What's missing is a comment saying "KVM changes this vendor id to
Intel when run on a VMX-capable host".

So what happens if you have a guest which worked fine under QEMU, then
breaks because it sees a changed vendor id when you run it under KVM?
I guess the guest, being Windows, tells you to make a phone call to
Microsoft because you changed the CPU, if you're out of luck.

It looks like it could be a KVM mis-feature.

Is the changing vendor id actually useful for anything, without
updating the family/model/stepping at the same time?  E.g. does it
make some guest things work, even though other parts of the cpuid
don't reflect the host?

> Another approach is to switch model to 3 along with vendor_id in KVM
> but because qemu is already using 6/3/3 when emulating a 32bit x86
> hardware, I don't think this is simpler and more consistent.

No, I'm happy with 6/3/3.  Like that part of the patch :-)

Although... there aren't any 64-bit Pentium Pros, so maybe 6/3/3 is a
bit... small? :-)

Now I'm just wondering why the vendor id needs to change dynamically,
if the other cpuid fields aren't changed.  If that's useful, then at
least a comment around the qemu64 CPU type to say that happens would
be good.  If that's not really a useful feature, then better not to do
it.

Maybe it's not useful any more, as "-cpu host" can be used instead (a
recent patch).  That had better not break Skype :-)

For Windows guests, which are sensitive to CPU changes, I prefer not
to change CPU type dynamically without the user knowing.  Even with
"-cpu host", I hope (eventually) the chosen CPU type will be stored
when saving/migrating and loaded exactly the same on other hosts, when
it's technically possible.

-- Jamie