Re: [Xen-devel] [PATCH v5 01/13] x86: reduce general stack alignment to 8

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Nov 30, 2018 at 02:03:29AM -0700, Jan Beulich wrote:
> >>> On 29.11.18 at 18:44, <wei.liu2@xxxxxxxxxx> wrote:
> > On Thu, Nov 08, 2018 at 09:05:45AM -0700, Jan Beulich wrote:
> >> --- a/xen/arch/x86/efi/Makefile
> >> +++ b/xen/arch/x86/efi/Makefile
> >> @@ -5,7 +5,11 @@ CFLAGS += -fshort-wchar
> >>  
> >>  boot.init.o: buildid.o
> >>  
> >> +EFIOBJ := boot.init.o compat.o runtime.o
> >> +
> >> +$(EFIOBJ): CFLAGS-stack-boundary := -mpreferred-stack-boundary=4
> > 
> > From gcc's manual on -mincoming-stack-boundary:
> > 
> > "Thus calling a function compiled with a higher preferred stack boundary
> > from a function compiled with a lower preferred stack boundary most
> > likely misaligns the stack." 
> > 
> > I notice runtime.o now has stack alignment of 2^4 while the rest of xen
> > has 2^3.
> > 
> > There is at least one example (efi_get_time) that could misalign the
> > stack. Is that okay?
> 
> It would not be okay if the runtime call machinery wouldn't force
> 32-byte alignment of the stack. See the declaration of struct
> efi_rs_state, an instance of which gets put on the stack of every
> function making runtime calls. Also note how this is no different
> from prior to this change, as explained by the comment in that
> structure declaration, except that instead of always running on
> a reliably mis-aligned stack we will now run on a mixture (hence
> the code [and stack] size savings mentioned in the description).

OK.

Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel