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

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Date:	Sun, 29 Mar 2015 21:36:33 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address

On Sat, Mar 28, 2015 at 10:11 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> >> $ ./timing_test64 iret
>> >> 10000 loops in 0.00344s = 343.90 nsec/loop for iret
>> >> 100000 loops in 0.01890s = 188.97 nsec/loop for iret
>> >> 1000000 loops in 0.08228s = 82.28 nsec/loop for iret
>> >> 10000000 loops in 0.77910s = 77.91 nsec/loop for iret
>> >>
>> >> This is the "same-ring interrupt return". ~230 cycles!  :(
>> >
>> > Ugh, that's really expensive! Why is that so? Same-ring irqs are
>> > supposedly a lot simpler.
>>
>> Descriptor checks for restored CS and SS,
>> checking canonical-ness of RIP,
>> supporting "return to TSS" (flags.NT bit),
>> "return to VM86" (flags.VM bit),
>> complex logic around restoring RFLAGS
>>   ("don't allow CPL3 to be able to disable interrupts...
>>   ...unless their flags.IOPL is 3." Gasp)
>> return to 16-bit code ("do not touch high 16 bits")
>>
>> All of this is a giant PITA to encode in microcode.
>
> 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.

Specifically, a CPL0-only IRET_FAST insn which uses the same stack layout
as IRET, but makes the following assumptions:

* The restored SS and CS are 0-based, 4G-limit segments.
   (as usual, in 64-bit mode limits are ignored).
* CS is read/execute, SS is read/write.
* The CPL to return to is equal to (CS & 3).

This would mean that IRET_FAST would not need to read descriptors
from GDT/LDT. It only needs to read values from stack.

It would be capable of returning both to CPL0 and CPL3 - iow,
usable for returning from interrupts both to userpace and kernelspace.

* FLAGS.NT is ignored (as if it is 0). IOW, no task returns.
* pt_regs->FLAGS.VM is not restored, but set to 0.
   IOW, no vm86.
* Extend this to other flags as well, if it makes return faster.
   We can have a separate code which restores AC,DF,IF,TF,RF,IOPL
   in the unlikely event they are "unusual". So it's okay
   if IRET_FAST just sets them to 0 (1 for IF).

The instruction would need a differentiator whether returned-to code
is 64-bit or 32-bit.
Then it probably can use the same approach SYSRET{O,L} uses:
with REX.W, return is to 64-bit; without it, return is to 32-bit.

Interrupt return then can check pt_regs->cs and use
IRETL_FAST if it is USER32_CS; use IRETQ_FAST if it is USER_CS
or KERNEL_CS; otherwise, fall back to slow but "universal" IRETQ.

Do we have contacts at Intel to petition for this? :D
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