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

[Daniel P. Berrangé]

On Thu, Feb 07, 2019 at 06:05:25PM +0400, Roman Bogorodskiy wrote:
>   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.

Yes, I agree. This is good as its the same behaviour as KVM.

How about we change the XML to

  <msrs unknown="ignore|fault"/>

Regards,
Daniel
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