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    SubjectRe: [PATCH v2 3/3] x86/pti/64: Remove the SYSCALL64 entry trampoline
    On Wed, Sep 05, 2018 at 02:31:28PM -0700, Andy Lutomirski wrote:
    > On Tue, Sep 4, 2018 at 12:04 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > > On Mon, Sep 03, 2018 at 03:59:44PM -0700, Andy Lutomirski wrote:

    > >> There is a possible alternative approach: we could instead move the
    > >> trampoline within 2G of the entry text and make a separate copy for
    > >> each CPU. Then we could use a direct jump to rejoin the normal
    > >> entry path.
    > >
    > > Can we have a few words on why this solution and not this alternative? I
    > > mean, you raise the possibility, but then surely you chose not to
    > > implement that. Might as well share that with us.
    >
    > I can give some pros and cons. With the other approach:
    >
    > - We avoid a pipeline stall.
    > - We execute from an extra page and read from another extra page
    > during the syscall. (The latter is because we need to use a relative
    > addressing mode to find sp1 -- it's the same *cacheline* we'd use
    > anyway, but we're accessing it using an alias, so it's an extra TLB
    > entry.)
    > - We use more memory. This would be one page per CPU for a simple
    > implementation and 64-ish bytes per CPU or one page per node for a
    > more complex implementation.
    > - More code complexity.
    >
    > I'm not convinced this is a good tradeoff.

    Fair enough, thanks!

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