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Re: [Qemu-devel] Re: [PATCH 3/4] mask out forbidden cpuid features with


From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: [PATCH 3/4] mask out forbidden cpuid features with kvm ioctl
Date: Tue, 03 Feb 2009 15:36:12 -0600
User-agent: Thunderbird 2.0.0.19 (X11/20090105)

Avi Kivity wrote:
Anthony Liguori wrote:
Glauber Costa wrote:
KVM has a (so far unused) ioctl to inform userspace
about supported cpuid bits in the current host.

The lack of this kind of checking can lead to bugs in which
a cpuid bit is exposed but not supported by the host kernel
(an example is fedora bug https://bugzilla.redhat.com/show_bug.cgi?id=481274)

The current host_cpuid code is replaced by this general check.

Signed-off-by: Glauber Costa <address@hidden>

So what do we do about live migration?

We need a way to completely specify cpuid from the command line. The management utility would figure out what is supported (driven by both cpu capabilities and installed kvm capabilities) and fgure out a greatest common denominator, or something.

What is the default case? Pass everything through or pick a reasonable small, constant subset? That is, is -cpuid a union or an intersection.

Regards,

Anthony Liguori

Does KVM mask a set of bits unconditionally?

The kernel doesn't touch cpuid (except nx due to an historical mistake).

Before, we were masking unconditionally which should insulate migration reasonable well.

That suits migration, but not other uses of qemu.






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