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    SubjectRe: [PATCH v6] x86: load FPU registers on return to userland
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    On 1/15/19 4:44 AM, David Laight wrote:
    > Once this is done it might be worth while adding a parameter to
    > kernel_fpu_begin() to request the registers only when they don't
    > need saving.
    > This would benefit code paths where the gains are reasonable but not massive.
    >
    > The return value from kernel_fpu_begin() ought to indicate which
    > registers are available - none, SSE, SSE2, AVX, AVX512 etc.
    > So code can use an appropriate implementation.
    > (I've not looked to see if this is already the case!)

    Yeah, it would be sane to have both a mask passed, and returned, say:

    got = kernel_fpu_begin(XFEATURE_MASK_AVX512, NO_XSAVE_ALLOWED);

    if (got == XFEATURE_MASK_AVX512)
    do_avx_512_goo();
    else
    do_integer_goo();

    kernel_fpu_end(got)

    Then, kernel_fpu_begin() can actually work without even *doing* an XSAVE:

    /* Do we have to save state for anything in 'ask_mask'? */
    if (all_states_are_init(ask_mask))
    return ask_mask;

    Then kernel_fpu_end() just needs to zero out (re-init) the state, which
    it can do with XRSTORS and a careful combination of XSTATE_BV and the
    requested feature bitmap (RFBM).

    This is all just optimization, though.

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