Re: [Xen-devel] [PATCH mini-os enhancements for vtpm 7/8] add floating point and sse to mini-os

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

Ian Campbell, le Tue 18 Sep 2012 08:38:23 +0100, a écrit :
> On Tue, 2012-09-18 at 08:31 +0100, Samuel Thibault wrote:
> > Ian Campbell, le Tue 18 Sep 2012 08:21:18 +0100, a écrit :
> > > On Mon, 2012-09-17 at 23:04 +0100, Matthew Fioravante wrote:
> > > > This patch adds floating point and sse support to mini-os by
> > > > initializing the floating point unit and the see unit during domain 
> > > > boot up.
> > > > 
> > > > 
> > > > Signed of by: Matthew Fioravante matthew.fioravante@xxxxxxxxxx
> > > > 
> > > > diff --git a/extras/mini-os/arch/x86/setup.c
> > > > b/extras/mini-os/arch/x86/setup.c
> > > > --- a/extras/mini-os/arch/x86/setup.c
> > > > +++ b/extras/mini-os/arch/x86/setup.c
> > > > @@ -74,9 +74,28 @@ shared_info_t *map_shared_info(unsigned long pa)
> > > >      return (shared_info_t *)shared_info;
> > > >  }
> > > >  
> > > > +static inline void fpu_init(void) {
> > > > +    asm volatile("fninit");
> > > > +}
> > > > +
> > > > +#ifdef __SSE__
> > > 
> > > How and when is this symbol defined?
> > 
> > This is defined by the compiler if some -msse is enabled. This is the
> > default on i686/x86_64 at least.
> 
> Do we support processors/compilers without SSE or conversely do we
> require SSE support?
> 
> It sounds like either we need a runtime test or the compile time test is
> unnecessary.

If the code is compiled with -msse (and thus __SSE__ defined), the
compiler itself can emit sse code, so a runtime test will not be enough
to avoid a crash on a non-sse machine.

The compile test is however not strictly needed since the code is
provided in assembly. And it might be useful to enable sse anyway in
case some -msse code gets pulled. IIRC we only support SSE machines, but
I'm not 100% sure.

Samuel

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