Re: [PATCH v3 00/17] target/arm: Implement LVA, LPA, LPA2 features

[Daniel P . Berrangé]

On Tue, Mar 01, 2022 at 04:30:30PM +0000, Peter Maydell wrote:
> On Tue, 1 Mar 2022 at 16:28, Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > On Tue, Mar 01, 2022 at 04:16:25PM +0000, Peter Maydell wrote:
> > > On Tue, 1 Mar 2022 at 16:08, Peter Maydell <peter.maydell@linaro.org> 
> > > wrote:
> > > >
> > > > On Wed, 23 Feb 2022 at 22:31, Richard Henderson
> > > > <richard.henderson@linaro.org> wrote:
> > > > >
> > > > > Changes for v3:
> > > > >   * Update emulation.rst.
> > > > >   * Split out separate update to ID_AA64MMFR0.
> > > > >   * Hack for avocado.
> > > > >
> > > > > If the avocado hack isn't acceptable, perhaps just drop the
> > > > > last two patches for now?
> > > >
> > > > I think that given that there are Linux kernels out there
> > > > that won't boot if LPA2 is enabled, we should probably have
> > > > a -cpu command line option to disable it. Otherwise we might
> > > > get a bunch of "why did my kernel stop booting" bug reports.
> > >
> > > ...and should using a versioned machine type also default
> > > -cpu max to not enabling that? Not sure what x86 or other
> > > precedent is there.
> >
> > I don't recall us coming across an important scenario where a guest
> > would fail to boot when we /enable/ a given CPU feature on x86,
> > requiring us to hide it from -cpu max/host.
> >
> > Assuming the QEMU/KVM implementation of a CPU feature is correct
> > per the relevant spec, then artificially hiding it by default from
> > -cpu max feels questionable, as that penalizes non-buggy guest OS.
> 
> Yeah. It's just unfortunate that "buggy guest OS" here is
> "every Linux up to 5.11".

Lets say the lpa2 feature was tied so it only gets enabled in the
new 7.0 machine type version, even if KVM/QEMU could supports it
fine with any machine type version. If someone runs a VM with
Linux 5.6 with

  -cpu max -machine virt'

it is going to break.

Our choice is then between telling them to change their QEMU config to

   -cpu max,-lpa2 -machine virt

vs telling them to use

   -cpu max -machine virt-6.2

there's not much practical difference in those scenarios, for someone
deploying a new VM instance.

It would, however, benefit people who had already previously deployed
a VM with QEMU using a explicit versioned machine type. This would
be the case for anyone using libvirt, as libvirt always expands the
user's config to be fully versioned.

With regards,
Daniel
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