Re: [PATCH 06/10] OvmfPkg/Sec: Disable optimizations for TemporaryRamMigration

[Jordan Justen <jordan.l.justen@intel.com>]

On 2019-02-17 23:53:01, Ard Biesheuvel wrote:
> On Mon, 18 Feb 2019 at 05:12, Jordan Justen <jordan.l.justen@intel.com> wrote:
> >
> 
> This needs an explanation why optimization needs to be disabled.

I'm not sure this is required. The reason I added these patches is to
hopefully prevent the compiler from removing the frame pointer. We
adjust the frame pointer in the code, and that is a little sketchy if
the frame pointer isn't being used.

Unfortunately, it can reasonably be argued that the
TemporaryRamSupport PPI definition ultimately makes it unsafe to write
the migration code in C.

I tried reverting both the EmulatorPkg and OvmfPkg patches for
disabling the optimizations, and with my setup there was no impact. I
think there is a good change that we'd be pretty safe to just drop
these two patches to wait and see if someone encounters a situation
that requires it.

Ok, so based on this explanation, do you think I should add info to
the commit message and keep the patches, or just drop them?

Thanks,

-Jordan

> 
> > Contributed-under: TianoCore Contribution Agreement 1.1
> > Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
> > Cc: Laszlo Ersek <lersek@redhat.com>
> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > Cc: Anthony Perard <anthony.perard@citrix.com>
> > Cc: Julien Grall <julien.grall@linaro.org>
> > ---
> >  OvmfPkg/Sec/SecMain.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/OvmfPkg/Sec/SecMain.c b/OvmfPkg/Sec/SecMain.c
> > index 46ac739862..86c22a2ac9 100644
> > --- a/OvmfPkg/Sec/SecMain.c
> > +++ b/OvmfPkg/Sec/SecMain.c
> > @@ -873,6 +873,13 @@ SecStartupPhase2(
> >    CpuDeadLoop ();
> >  }
> >
> > +#ifdef __GNUC__
> > +#pragma GCC push_options
> > +#pragma GCC optimize ("O0")
> > +#else
> > +#pragma optimize ("", off)
> > +#endif
> > +
> >  EFI_STATUS
> >  EFIAPI
> >  TemporaryRamMigration (
> > @@ -946,3 +953,8 @@ TemporaryRamMigration (
> >    return EFI_SUCCESS;
> >  }
> >
> > +#ifdef __GNUC__
> > +#pragma GCC pop_options
> > +#else
> > +#pragma optimize ("", on)
> > +#endif
> 
> I can't tell from the context if this is the end of the file, but if
> it is not, aren't you turning on optimization here for non-GCC even if
> it was not enabled on the command line to begin with?