RE: [PATCH v5] x86: use builtins to read eflags

[David Laight <David.Laight@ACULAB.COM>]

From: Linus Torvalds
> Sent: 18 March 2022 18:19
> 
> On Fri, Mar 18, 2022 at 10:59 AM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > I generally agree.  In this particular case, though, I will keep using
> > the builtin in tools/testing/selftests/x86/helpers.h unless we actually
> > hit breakage.  This is because this is *user* code, it is compiled with
> > the redzone enabled, and the asm code to do the right thing when the
> > redzone is enabled is too hairy for me to want to deal with it.
> 
> Yeah, redzoning is a problem for "pushf".
> 
> Or rather, it's not the redzoning itself, but the fact that the
> compiler might use the word under the stack for random other things,
> and the pushf will then corrupt some local variable storage.
> 
> I think it would be lovely to solve that in inline asm itself some way
> - by marking the stack pointer clobbered or something.

Something that generates:
	mov	%rax,-8(%rsp)
	pushf
	xchg	%rax,0(%rsp)
	add	%rsp,8
should work with or without a redzone.
Will be a bit slower :-(

	David

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