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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 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;
    }

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