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

[Roman Bogorodskiy]

  Daniel P. Berrangé wrote:

> 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.

From what I can see by briefly looking at the code, it applies to both
reads and writes:

https://svnweb.freebsd.org/base/head/usr.sbin/bhyve/bhyverun.c?revision=343642&view=markup#l520

Check two functions below this line.

> > 
> > 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
> -- 
> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

Roman Bogorodskiy
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 455 bytes
Desc: not available
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20190207/3c36a3ab/attachment-0001.sig>