Re: [PATCH v2 3/3] x86/pti/64: Remove the SYSCALL64 entry trampoline

[Peter Zijlstra]

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@xxxxxxxxxxxxx> 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!