Re: [Qemu-devel] aarch64 & gdb: warning: while parsing target description (at line 1): Could not load XML document "arm-core.xml"

["Richard W.M. Jones" <rjones@redhat.com>]

On Thu, Aug 07, 2014 at 12:35:27PM +0100, Peter Maydell wrote:
> On 7 August 2014 12:29, Richard W.M. Jones <rjones@redhat.com> wrote:
> > On Thu, Aug 07, 2014 at 12:18:49PM +0100, Peter Maydell wrote:
> >> you didn't select a 32 bit CPU either explicitly or by default
> >> on the QEMU command line? Note that '-machine type=virt'
> >> defaults to a Cortex-A15 even in qemu-softmmu-aarch64
> >> (this is unfortunate but fallout from the fact that we started
> >> the virt model with the A15. Maybe we should make it not
> >> have a default and require a CPU specification...)
> >
> > Yes, this is indeed the case.  It also explains why I could get TCG
> > working at all until I added -cpu cortex-a57.  Now it is working.
> >
> > I really think this choice of cpu_model = "cortex-a15" for -M virt is
> > a poor one.  It should always default to something working.
> 
> cortex-a15 *does* work, it's just a 32 bit CPU. It will function
> exactly the same as if you asked for an A15 in qemu-system-arm.
> Obviously if you try to feed it an AArch64 kernel it will behave
> the same way as if you'd tried to boot an AArch64 kernel on
> A15 hardware, so don't do that.

Can't a 32 bit kernel run on -cpu cortex-a5x?  (I've not tried.)

> > Currently libguestfs has to pass -cpu host when KVM is detected, and
> > -cpu cortex-a57 otherwise.  However we cannot be sure that KVM is 100%
> > working and that we won't fall back to TCG, which means the -cpu
> > parameter could be wrong and we would fail to launch the appliance.
> > Having the correct defaults would prevent this unnecessary failure.
> 
> This isn't possible, I'm afraid. TCG will never support "-cpu host",
> and whether any particular specific CPU type is supported by
> the kernel for KVM depends on the host hardware. (We don't
> want to default to "-cpu host if KVM" because it means defaulting
> to "migration won't work".)

We use -cpu host on KVM in order to avoid parsing /proc/cpuinfo to
find the right CPU model and flags.  Parsing /proc/cpuinfo is a
minefield because it differs on every architecture, and even within
architectures.

Note libguestfs doesn't care about migration at all.  We do care that
we can reliably run our appliance.

What we really want is -cpu best-possible-and-don't-fail.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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