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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Mon, 18 Feb 2019 at 10:08, Jordan Justen <jordan.l.justen@intel.com> wrote:
>
> 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?
>

I think 'little sketchy' is an understatement here (as is
setjmp/longjmp in general), but it is the reality we have to deal with
when writing startup code in C. Looking at the code, I agree that the
fact that [re]bp is assigned directly implies that we should not
permit it to be used as a general purpose register, especially when
you throw LTO into the mix, which could produce all kinds of
surprising results when it decides to inline functions being called
from here.

For GCC/Clang, I don't think it is correct to assume that changing the
optimization level will result in -fno-omit-frame-pointer to be set,
so I'd prefer setting that option directly, either via the pragma, or
for the whole file.

For MSVC, I have no idea how to tweak the compiler to force it to emit
frame pointers.


> >
> > > 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?