Re: [Qemu-devel] [PATCH 0/3] target-i386: Don't trigger "check" warnings by default in KVM mode

[Eduardo Habkost]

On Mon, Nov 02, 2015 at 12:37:24PM +0100, Paolo Bonzini wrote:
> On 30/10/2015 20:13, Eduardo Habkost wrote:
> > On Fri, Oct 30, 2015 at 05:00:57PM -0200, Eduardo Habkost wrote:
> >> The x86 change to make "check" mode be enabled by default made QEMU print a
> >> warning in the default case if running in an Intel host:
> >>
> >>   $ qemu-system-x86_64 -machine pc,accel=kvm
> >>   warning: host doesn't support requested feature: 
> >> CPUID.80000001H:ECX.sse4a [bit 6]
> >>
> >> Fix this by not enabling sse4a in qemu64 when in KVM mode.
> >>
> >> The long term solution will probably involve creating separate "tcg64" and
> >> "kvm64" CPU models as defaults, so we can finally choose completely 
> >> diffferent
> >> defaults in the KVM- and TCG-specific models without making the
> >> kvm_default_props list grow too mcuh.
> > 
> > Note that we can have a much simpler solution to this: disabling SSE4A
> > on qemu64 completely, even on TCG. This way we can use PC_COMPAT_2_4
> > instead of the kvm_default_props compat hack.
> > 
> > We can reenable SSE4A for TCG later (more exactly, we could enable
> > everything from TCG_*_FEATURES) when we implement the kvm64/tcg64
> > solution.
> > 
> > What do you think?
> 
> I would prefer that to this series.

I just got a report from David that the lack of ABM on Sandy Bridge
hosts will trigger the warning, too. I guess we don't want to require a
Haswell CPU to run the default CPU model with KVM, so ABM would be
disabled by default in KVM mode too.

We really should make the defaults different in TCG and KVM mode. The
question is what to do about the TCG defaults in QEMU 2.5.

I propose we remove ABM and SSE4E from qemu64 in 2.5, and implement a
proper mechanism to have different KVM and TCG defaults after 2.5. We
never pretended that qemu64 had all TCG-supported features enabled
anyway: the last time the feature set of qemu64 was changed was in 2009
(when we added ABM).

-- 
Eduardo