Re: [PATCH] x86/asm/entry/64: better check for canonical address

[Andy Lutomirski]

On Tue, Mar 31, 2015 at 9:43 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote:
>
>> > I guess they could optimize it by adding a single "I am a modern
>> > OS executing regular userspace" flag to the descriptor [or
>> > expressing the same as a separate instruction], to avoid all that
>> > legacy crap that won't trigger on like 99.999999% of systems ...
>>
>> Yes, that would be a useful addition. Interrupt servicing on x86
>> takes a non-negligible hit because of IRET slowness.
>
> But ... to react to your other patch: detecting the common easy case
> and doing a POPF+RET ourselves ought to be pretty good as well?
>
> But only if ptregs->rip != the magic RET itself, to avoid recursion.
>
> Even with all those extra checks it should still be much faster.
>

I have a smallish preference for doing sti;ret instead, because that
keeps the funny special case entirely localized to the NMI code
instead of putting it in the IRQ exit path.  I suspect that the
performance loss is at most a cycle or two (we're adding a branch, but
sti itself is quite fast).

That being said, I could easily be convinced otherwise.  We'd
certainly get better code coverage if the special case were in the IRQ
exit path, since it would presumably happen fairly frequently.  Heck,
executing write(anything, BAD_USER_POINTER, 1) in a tight loop would
probably hit that case very quickly if there's any network traffic.

--Andy
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