Re: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code

[H. Peter Anvin]

On 9/7/21 9:42 PM, H. Peter Anvin wrote:
> On 9/7/21 6:38 PM, H. Peter Anvin wrote:
> > On 9/2/21 3:50 AM, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > >
> > > There is no constrain/limition to force native_load_gs_index() to be in
> > > ASM code.
> >
> > Hi,
> >
> > First of all, let me say I really like your patchset, and I will try to
> > review it in detail ASAP (most of the initial read pass looks very sane
> > to me.
> >
> > However, I would like to object in part this specific patch. It adds a
> > fair bit of extra code to the exception path, and adds jumps between
> > files which makes the code much harder to read.
> >
> > You end up doing one swapgs in assembly and one in C, which would seem
> > to be a very good indication that really isn't an improvement.
> >
> > Note that this entire sequence is scheduled to be obsoleted by a single
> > atomic hardware instruction, LKGS, which will replace ALL of
> > native_load_gs_index(); it will no longer be necessary even to disable
> > interrupts as there is no non-atomic state. In that sense, doing this as
> > an out-of-line C function (with some inline assembly) is great, because
> > it makes it far easier to use LKGS as an alternative; the only (small)
> > disadvantage is that it ends up clobbering a lot of registers
> > unnecessarily (in assembly it can be implemented clobbering only two
> > registers; one if one uses pushf/popf to save the interrupt flag.)
>
> OK, here is a version which actually compiles:

... slightly shorter and minimally better compiled code ...

noinstr void native_load_gs_index(unsigned int selector)
{
	unsigned long flags;

	local_irq_save(flags);
	native_swapgs();
do_mov_gs:
	asm_volatile_goto("1: mov %[seg],%%gs\n"
			  "2:\n"
			  _ASM_EXTABLE(1b,%l[bad_seg])
			  : : [seg] "r" (selector) : : bad_seg);
	alternative("", "mfence", X86_BUG_SWAPGS_FENCE);
	native_swapgs();
	local_irq_restore(flags);
	return;

bad_seg:
	/* The exception dispatch will have restored kernel GS */
	native_swapgs();
	alternative_input("", "mov %[seg],%%gs",
			  X86_BUG_NULL_SEG, [seg] "r" (__USER_DS));
	selector = 0;
	goto do_mov_gs;
}