RE: [patch v3 16/16] btrace, x86: restrict to Atom

["Metzger, Markus T" <markus.t.metzger@intel.com>]

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> -----Original Message-----
> From: Metzger, Markus T
> Sent: Tuesday, August 14, 2012 3:42 PM
> To: Mark Kettenis
> Cc: gdb-patches@sourceware.org; markus.t.metzger@gmail.com; jan.kratochvil@redhat.com; palves@redhat.com;
> tromey@redhat.com
> Subject: RE: [patch v3 16/16] btrace, x86: restrict to Atom
>
> > -----Original Message-----
> > From: Mark Kettenis [mailto:mark.kettenis@xs4all.nl]
> > Sent: Tuesday, August 14, 2012 3:27 PM
>
> Thanks for your feedback!
>
>
> > > +int
> > > +i386_linux_supports_btrace (void)
> > > +{
> > > +  unsigned int cpuid, model, family;
> > > +
> > > +  if (!linux_supports_btrace ())
> > > +    return 0;
> > > +
> > > +  __asm__ __volatile__ ("movl   $1, %%eax;"
> > > +                 "cpuid;"
> > > +                 : "=a" (cpuid)
> > > +                 :: "%ebx", "%ecx", "%edx");
> > > +
>
> [...]
>
> > You can't have this function in a -tdep.c, since this file needs to be
> > compilable everywhere (not just on i386/amd64).  But even in a -nat.c
> > file you shouldn't use inline assembly, since it isn't a standardized
> > C feature.  I can only guess why you want to restrict this feature to
> > Atom only, but I suspect that the proper solution is to query the
> > target whether branch tracing is supported.  That means the -nat.c
> > code should have code to query for support in the Linux kernel.
>
> I agree that it would be better to have this check done in the kernel. The problem is that such a patch will take very long before
> it becomes available in distributions. And even then, we would need to handle kernels without the patch, so we would need
> something
> similar to this in gdb, as well.
>
> I took the cpuid bits from gdb/go32-nat.c. I have not seen any compilation guard around it.
>
> I thought that the i386-tdep files are meant to contain i386 specific code, that's why I put the code there. Where would be the
> right place to put such code?
>
> Thanks,
> Markus.
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