[libvirt] [PATCH 0/3] bhyve: implement MSRs ignore unknown writes feature

[Daniel P. Berrangé]

On Wed, Feb 06, 2019 at 04:00:06PM -0500, Cole Robinson wrote:
> On 1/25/19 12:54 PM, Roman Bogorodskiy wrote:
> > 
> > Roman Bogorodskiy (3):
> >   conf: introduce 'msrs' feature
> >   bhyve: implement MSRs ignore unknown writes feature
> >   news: document bhyve msrs feature
> > 
> >  docs/drvbhyve.html.in                         | 16 +++++++++
> >  docs/formatdomain.html.in                     |  1 +
> >  docs/news.xml                                 | 11 ++++++
> >  docs/schemas/domaincommon.rng                 | 14 ++++++++
> >  src/bhyve/bhyve_command.c                     |  4 +++
> >  src/conf/domain_conf.c                        | 33 +++++++++++++++++
> >  src/conf/domain_conf.h                        |  8 +++++
> >  src/qemu/qemu_domain.c                        |  1 +
> >  .../bhyvexml2argvdata/bhyvexml2argv-msrs.args | 10 ++++++
> >  .../bhyvexml2argv-msrs.ldargs                 |  3 ++
> >  .../bhyvexml2argvdata/bhyvexml2argv-msrs.xml  | 26 ++++++++++++++
> >  tests/bhyvexml2argvtest.c                     |  1 +
> >  .../bhyvexml2xmlout-msrs.xml                  | 36 +++++++++++++++++++
> >  tests/bhyvexml2xmltest.c                      |  1 +
> >  14 files changed, 165 insertions(+)
> >  create mode 100644 tests/bhyvexml2argvdata/bhyvexml2argv-msrs.args
> >  create mode 100644 tests/bhyvexml2argvdata/bhyvexml2argv-msrs.ldargs
> >  create mode 100644 tests/bhyvexml2argvdata/bhyvexml2argv-msrs.xml
> >  create mode 100644 tests/bhyvexml2xmloutdata/bhyvexml2xmlout-msrs.xml
> > 
> 
> The code looks fine to me but this needs a bit more context.
> Particularly 'ignore' is a bit ambiguous here so I had to dig
> 
> kvm+linux has arguably always ignored unknown MSRs but historically it
> would print an error in host dmesg which often confused users quite a
> bit. In the bhyve case it seems that unknown MSRs cause the VM to
> essentially crash which is a pretty intense reaction. This option
> disables that crashing behavior. So for this feature to be really
> descriptive it would be <msrs crashOnUnknown='yes|no' /> or something
> like that

That is not actually the case for KVM.   When KVM sees an unhandled
MSR it will inject a GPF to the guest, which will usually cause the
guest to crash. KVM does this GPF injection for both reads and writes
of unknown MSRs.

There is an opt-in kvm.ko  option "ignore-msrs" which tells KVM
to silently ignore unknown MSRs instead. Ideally the KVM option
would be exposed to QEMU as an option, since when people do need
it, they usually only need it for a single guest.

> The bhyve man page says:
> 
> https://www.freebsd.org/cgi/man.cgi?query=bhyve&sektion=8
> 
> -w    Ignore accesses to unimplemented Model Specific Registers
>       (MSRs). This is intended for debug purposes.

I'm curious whether this applies to boths reads & writes of unknown
MSRs, or just to writes.  '-w' naming suggests the latter, but the
descrption "accesses" would suggest reads too.

> 
> Calling it 'intended for debug purposes' also makes me question whether
> this should be encoded in the libvirt XML or just worked around with
> commandline passthrough
> 
> (IMO To be friendly to users this option should be enabled by default
> but obviously that's against the intention of bhyve devs...)

Enabling it by default is not a good idea as it can lead to silently
incorrect guest OS behaviour. That could lead to all manner of bad
things depending on the MSR in question.

> ccing danpb to see if he has any thoughs about exposing this in the XML

My only thought is the naming & whether we have one attribute for
ignoring reads & writes, or separate attributes.

Regards,
Daniel
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