Re: [kvm-unit-tests PATCH] x86/memory: pass host clwb and clflushopt support information

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Dec 19, 2017 at 10:54:16AM +0800, Haozhong Zhang wrote:
> On 12/18/17 16:36 -0200, Eduardo Habkost wrote:
> > On Tue, Dec 12, 2017 at 04:35:24PM +0800, Haozhong Zhang wrote:
> > > Intel VMX cannot intercept guest clwb and clflushopt. When clwb and
> > > clflushopt are not exposed in guest cpuid, clwb and clflushopt
> > > instructions in this test case can still succeed without #UD on the
> > > host CPU which has clwb and clflushopt support, though failures with
> > > UD are expected.
> > > 
> > > In order to avoid false alarms in such cases, introduce the following
> > > two arguments "has_clwb" and "has_clflushopt" to allow users to
> > > specify whether clwb and clflushopt are supported on the host CPU.
> > > 
> > > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > > ---
> > >  x86/memory.c | 29 +++++++++++++++++++++++++++--
> > >  1 file changed, 27 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/x86/memory.c b/x86/memory.c
> > > index cd1eb46..03ff7d3 100644
> > > --- a/x86/memory.c
> > > +++ b/x86/memory.c
> > > @@ -23,10 +23,29 @@ static void handle_ud(struct ex_regs *regs)
> > >  	regs->rip += isize;
> > >  }
> > >  
> > > +/*
> > > + * Intel VMX cannot intercept guest clwb and clflushopt. When clwb and
> > > + * clflushopt are not exposed in guest cpuid, clwb and clflushopt
> > > + * instructions in this test case can still succeed without #UD on
> > > + * the host CPU which has clwb and clflushopt support. In order to avoid
> > > + * false alarms in such cases, introduce the following two arguments
> > > + * to allow users to specify whether clwb and clflushopt are supported on
> > > + * the host CPU:
> > > + * - has_clwb:       indicates clwb is supported on the host CPU
> > > + * - has_clflushopt: indicates clflushopt is supported on the host CPU
> > > + */
> > 
> > Why not simply use "-cpu host" to make sure the guest CPUID flags
> > match host CPUID?
> >
> 
> Can I understand that testing these two cases with host/guest CPUID
> mismatch (specially clwb and clflushopt flags) is invalid? If yes,
> please ignore this patch.

I wouldn't say it's invalid to test what happens when the host
and guest CPUID don't match.  The question is: is it useful to do
so?  Are we testing different code paths when we do that?

The inability to trigger #UD if the host CPUID includes the flag
sounds like a bug/limitation we would like to get rid of as soon
as hardware allow us to, and not a feature we need to test for.

What's the right way to ensure memory.flat is always tested using
"-cpu host"?

-- 
Eduardo