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

[Lai Jiangshan]

On 2021/9/8 13:00, H. Peter Anvin wrote:
> 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.

Hello

Thank you for your review.

> > > 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.

I tried putting all code into a single C function.
But I didn't know how to use a C-label in _ASM_EXTABLE and then I split it.

Your code is much better.

Thanks
Lai


> > > 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;
> }