Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

Cyrill Gorcunov
2018-11-08T21:31:26Z

On Thu, Nov 08, 2018 at 01:22:54PM -0800, Andy Lutomirski wrote:
> > >
> > > Why are these __packed? It seems like it'll generate bad code for no
> > > obvious purpose.
> >
> > That prevents any possibility that the compiler will insert padding, although in
> > 64-bit kernel this should not happen to either struct. Also all xstate
> > components here are packed.
> >
>
> They both seem like bugs, perhaps. As I understand it, __packed
> removes padding, but it also forces the compiler to expect the fields
> to be unaligned even if they are actually aligned.

How is that? Andy, mind to point where you get that this
attribute forces compiler to make such assumption?