* unconditional CPUID propagation?
@ 2011-08-03 9:41 Andre Przywara
2011-08-04 14:07 ` Marcelo Tosatti
0 siblings, 1 reply; 2+ messages in thread
From: Andre Przywara @ 2011-08-03 9:41 UTC (permalink / raw)
To: Avi Kivity, Anthony Liguori; +Cc: KVM list, Alexander Graf
Hi,
while looking through the code I found commit
f79116867ec80ed5d1d10043a3fd9ac8afd182c1 (upstream QEMU: enable SMEP)
which unconditionally propagates the bits from CPUID leaf 0x7 to the
guest. Though there is the KVM module in the line, this currently
whitelists three feature bits.
Doesn't that break migration? The result of the CPUID instruction the
guess issues only depends on the host and the KVM module's policy, not
on the CPU model QEMU uses. So I guess migrating from a newer CPU to an
older one breaks despite a rather conservative CPU model has been chosen
intentionally by the user.
The same is probably true for the VIA CPUID leaf.
Is that considered OK now or is that a bug? Shall the new feature bits
be made known to QEMU like the other ones on only enabled explicitly
(+smep) or by -cpu host?
I can make a patch for that if that is the right way to address this.
Regards,
Andre.
--
Andre Przywara
AMD-OSRC (Dresden)
Tel: x29712
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: unconditional CPUID propagation?
2011-08-03 9:41 unconditional CPUID propagation? Andre Przywara
@ 2011-08-04 14:07 ` Marcelo Tosatti
0 siblings, 0 replies; 2+ messages in thread
From: Marcelo Tosatti @ 2011-08-04 14:07 UTC (permalink / raw)
To: Andre Przywara; +Cc: Avi Kivity, Anthony Liguori, KVM list, Alexander Graf
On Wed, Aug 03, 2011 at 11:41:42AM +0200, Andre Przywara wrote:
> Hi,
>
> while looking through the code I found commit
> f79116867ec80ed5d1d10043a3fd9ac8afd182c1 (upstream QEMU: enable
> SMEP) which unconditionally propagates the bits from CPUID leaf 0x7
> to the guest. Though there is the KVM module in the line, this
> currently whitelists three feature bits.
> Doesn't that break migration? The result of the CPUID instruction
> the guess issues only depends on the host and the KVM module's
> policy, not on the CPU model QEMU uses. So I guess migrating from a
> newer CPU to an older one breaks despite a rather conservative CPU
> model has been chosen intentionally by the user.
> The same is probably true for the VIA CPUID leaf.
>
> Is that considered OK now or is that a bug? Shall the new feature
> bits be made known to QEMU like the other ones on only enabled
> explicitly (+smep) or by -cpu host?
> I can make a patch for that if that is the right way to address this.
Or if the CPU type supports it, yes.
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2011-08-03 9:41 unconditional CPUID propagation? Andre Przywara
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