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    SubjectRe: [RFC PATCH 5/6] x86/alternatives: use temporary mm for text poking
    On Wed, Aug 29, 2018 at 2:28 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > On Wed, Aug 29, 2018 at 01:11:46AM -0700, Nadav Amit wrote:

    >> + pte_clear(poking_mm, poking_addr, ptep);
    >> +
    >> + /*
    >> + * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
    >> + * as it also flushes the corresponding "user" address spaces, which
    >> + * does not exist.
    >> + *
    >> + * Poking, however, is already very inefficient since it does not try to
    >> + * batch updates, so we ignore this problem for the time being.
    >> + *
    >> + * Since the PTEs do not exist in other kernel address-spaces, we do
    >> + * not use __flush_tlb_one_kernel(), which when PTI is on would cause
    >> + * more unwarranted TLB flushes.
    >> + */
    >
    > yuck :-), but yeah.

    I'm sure we covered this ad nauseum when PTI was being developed, but
    we were kind of in a rush, so:

    Why do we do INVPCID at all? The fallback path for non-INVPCID
    systems uses invalidate_user_asid(), which should be faster than the
    invpcid path. And doesn't do a redundant flush in this case.

    Can we just drop the INVPCID? While we're at it, we could drop
    X86_FEATURE_INVPCID_SINGLE entirely, since that's the only user.

    --Andy

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