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SubjectRe: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code
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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@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:
>

... 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;
}

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