lkml.org 
[lkml]   [2021]   [Sep]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code
From
Date


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@linux.alibaba.com>
>>
>> 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:

(It makes me wonder if it would be useful to have an _ASM_EXTABLE_GOTO()
macro to highlight the use of an asm goto C label.)

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:
selector = 0;

/* The exception dispatch will have restored kernel GS */
native_swapgs();

if (static_cpu_has_bug(X86_BUG_NULL_SEG))
asm volatile("mov %[seg],%%gs"
: : [seg] "r" (__USER_DS));
goto do_mov_gs;
}

\
 
 \ /
  Last update: 2021-09-08 06:44    [W:6.701 / U:0.012 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site