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

Thomas Gleixner
2018-09-08T06:33:43Z

On Fri, 7 Sep 2018, Linus Torvalds wrote:
> On Fri, Sep 7, 2018 at 12:54 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > > - 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.)
> >
> > Ok, but is this really an issue with PTI?
>
> I'd expect it to be *more* of an issue with PTI, since you're already
> wasting TLB entries due to the whole "two different page tables".
>
> Sure, address space ID's save you from reloading them all the time,
> but don't help with capacity.
>
> But yeah, in the sense of "with PTI, all kernel entries are slow
> anyway, so none of this matters" is probably correct in a very real
> sense.

Unfortunately yes. As Andy pointed out the alternative approach would trade
an avoided pipeline stall with an extra TLB.

> That said, the real reason I like Andy's patch series is that I think
> it's simpler than the alternatives (including the current setup). No
> subtle mappings, no nothing. It removes a lot more lines than it adds,
> and half the lines that it *does* add are comments.

I agree and in fact I had the series already queued for merging, I just
wanted to get the full picture and add some of that information to the
changelog.

> Virtual mapping tricks may be cool, but in the end, not having to use
> them is better still, I think.

Sure, unless they provide a real value. The maybe smaller leak of address
information is probably not worth it.

Thanks,

tglx