Re: [RFC/HACK] x86: Fast return to kernel

[Andy Lutomirski]

On Fri, May 2, 2014 at 2:01 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, May 2, 2014 at 1:30 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>
>> So what about manipulating the stack so that the popf does not enable
>> interrupts and do an explicit sti to get the benefit of the
>> one-instruction shadow ?
>
> That's what I already suggested in the original "I don't think popf
> works" email.
>
> It does get more complex since you now have to test things (there are
> very much cases where we get page faults and other exceptions with
> interrupts disabled), but it shouldn't be much worse.
>
> Btw, Andy, why did you do "popq %rsp"? That just looks crazy. If the
> stack isn't contiguous, the subsequent "popf" couldn't have worked
> anyway. And I bet it screws with the stack engine. So you should just
> have done something like "addq $16,%rsp" or whatever the constant ends
> up being.

Because otherwise I'd have to keep track of whether it's a zeroentry
or an errorentry.  I can't stuff the offset in a register without even
more stack hackery, since there are no available registers there.  I
could split the whole thing into two code paths, I guess.

--Andy
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