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

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

On 2019-02-19 14:50:13, Brian J. Johnson wrote:
> On 2/18/19 3:32 AM, Ard Biesheuvel wrote:
> > 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.
> > 
> 
> https://docs.microsoft.com/en-us/cpp/build/reference/oy-frame-pointer-omission?view=vs-2017

Hmm, and based on:

https://docs.microsoft.com/en-us/cpp/preprocessor/optimize?view=vs-2017

#pragma optimize ("", off)

...includes the "y" option.

This 2nd page seems a little confused, as it documents "y" as
"Generate frame pointers on the program stack", while the 1st page
says "Suppresses creation of frame pointers on the call stack".

I think the "suppress" is more accurate as it makes more sense that
suppressing the frame pointer gives better optimization opportunities.

Anyway, I think that means that `#pragma optimize ("", off)` does what
we want on MSVC to force frame pointers to be used.

-Jordan