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Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host |
Date: |
Wed, 24 Jun 2009 22:39:44 +0100 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
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
- [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Andrea Arcangeli, 2009/06/23
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Jamie Lokier, 2009/06/24
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Filip Navara, 2009/06/24
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Andrea Arcangeli, 2009/06/24
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host,
Jamie Lokier <=
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Andrea Arcangeli, 2009/06/24
- [Qemu-devel] KVMs default CPU type (was: allow sysenter on 32bit guests running on vmx host), Andre Przywara, 2009/06/25
- Re: [Qemu-devel] KVMs default CPU type, Avi Kivity, 2009/06/25
- Re: [Qemu-devel] KVMs default CPU type (was: allow sysenter on 32bit guests running on vmx host), Andrea Arcangeli, 2009/06/25
- Re: [Qemu-devel] KVMs default CPU type (was: allow sysenter on 32bit guests running on vmx host), Andrea Arcangeli, 2009/06/25
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Paul Brook, 2009/06/25
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Andrea Arcangeli, 2009/06/25
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Paul Brook, 2009/06/25
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Andrea Arcangeli, 2009/06/25
- Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host, Paul Brook, 2009/06/25