[MODERATED] Re: [PATCH v2 3/8] MDSv2 5

[Linus Torvalds <torvalds@linux-foundation.org>]

Honestly, this looks completely bogus.

On Mon, Dec 10, 2018 at 2:37 PM speck for Andi Kleen
<speck@linutronix.de> wrote:
>
> +void clear_cpu_buffers_idle(void)
> +{
> +       if (cpu_smt_control != CPU_SMT_ENABLED)
> +               return;
> +       /* Has to be memory form, don't modify to use an register */
> +       alternative_input("",
> +               "pushq %[kernelds]; verw (%%rsp) ; addq $8,%%rsp \n",
> +               X86_FEATURE_MB_CLEAR,
> +               [kernelds] "i" (__KERNEL_DS));
> +}

So you have a non-inline function that

 - has a test to return early

 - does a *single* instruction

 - the instruction gets nop'ed out if it's not valid

all of which just *screams* "that's wrong" to me. Why isn't it inline
and a static branch?

The actual asm sequence also seems bad to me.

Why isn't that just using

        int val = __KERNEL_DS;

with the asm just doing

        "verw %[kernelds]" ..  [kernelds] "m" (val)

instead, letting gcc do the stack setup part.

Maybe none of this matters simply because it's in the idle case, but
the exact same issues happen for the kernel exit case.

And no, we're not doing that insane kernel exit software case by
default. Not without a whole lot of examples of horrid badness.

The data that system calls touch is basically already user data. Sure,
there may be kernel pointers etc there, but we've already accepted
those leaking locally for all the usual reasons that we have no
control over.

So the whole "let's add crazy long sequences to every kernel exit" is
not going to happen. Not without a lot more explanations.

                   Linus