Re: [Qemu-devel] [PATCH 1/6] kvm: Add KVM_GET_EMULATED_CPUID

[Eduardo Habkost]

On Sat, Sep 28, 2013 at 12:49:04PM +0200, Borislav Petkov wrote:
> On Fri, Sep 27, 2013 at 11:21:34AM -0300, Eduardo Habkost wrote:
> > The problem here is that "requested_features" doesn't include just
> > the explicit "+flag" flags, but any flag included in the CPU model
> > definition. See the "-cpu n270" example below.
> 
> Oh, you mean if requested_features would contain a flag included from
> the CPU model definition - a flag which we haven't requested explicitly
> - and if kvm emulates that flag, then it will get enabled?

Exactly. The code needs to filter/check all feature bits on the CPU, not
just the ones requested explicitly in the command-line.

[...]
> > [1] Maybe one source of confusion is that the existing code have two
> > feature-filtering functions doing basically the same thing:
> > filter_features_for_kvm() and kvm_check_features_against_host().  That's
> 
> Yes, and the first gets executed unconditionally and does the feature
> filtering,  right after the second has run in the kvm_enabled() branch.

This should be fixed, too: eventually "enforce" should work on TCG mode
as well.

> 
> > something we must clean up, and they should be unified. "enforce" should
> > become synonymous to "make sure filtered_features is all zeroes".  This
> > way, libvirt can emulate what 'enforce" does while being able to collect
> > detailed error information (which is not easy to do if QEMU simply
> > aborts).
> 
> Ok, maybe someone who's more knowledgeable with this code should do it -
> not me :)

I have added it to my TODO-list.  :-)

> 
> Also, there's another aspect, while we're here: now that QEMU emulates
> MOVBE with TCG too, how do we specify on the command line, which
> emulation should be used - kvm.ko or QEMU?

You can use accel={tcg,kvm} option on the "-machine" argument, e.g.
"-machine pc,accel=kvm". Or the "-enable-kvm" option.

-- 
Eduardo