linux-kernel - Re: [PATCH v2] x86: Return to kernel without IRET

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Date:	Sat, 03 May 2014 19:15:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2] x86: Return to kernel without IRET

We have to do that anyway to deal with 16- and 32-bit userspace return.

On May 3, 2014 5:31:41 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Sat, May 3, 2014 at 4:51 PM, Andy Lutomirski <luto@...capital.net>
>wrote:
>> On Sat, May 3, 2014 at 3:19 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 05/03/2014 04:24 AM, Steven Rostedt wrote:
>>>> On Fri, 02 May 2014 21:03:10 -0700
>>>> "H. Peter Anvin" <hpa@...or.com> wrote:
>>>>
>>>>>
>>>>> I'd really like to see a workload which would genuinely benefit
>before
>>>>> adding more complexity.  Now... if we can determine that it
>doesn't harm
>>>>> anything and would solve the NMI nesting problem cleaner than the
>>>>> current solution, that would justify things, too...
>>>>>
>>>>
>>>> As I stated before. It doesn't solve the NMI nesting problem. It
>only
>>>> handles page faults. We would have to implement this for breakpoint
>>>> return paths too. Is that a plan as well?
>>>>
>>>
>>> I would assume we would do it for *ALL* the IRETs.  There are only
>three
>>> IRETs in the kernel last I checked...
>>
>> I think we should carefully avoid doing it for returns from NMI,
>though :)
>>
>> If you want a realistic benchmark that will speed up, packet
>> forwarding might be a good place to look.
>
>Hmm.  I think my patch will blow up with EFI mixed mode if any EFI
>functions are called with interrupts enabled.  It may also blow up
>with when suspending or doing other BIOS things like that.  It should
>probably check the actual value of CS as opposed to just the CPL.
>
>I'm not sure what's happening with the alternate GDT in the EFI stuff.
>
>--Andy

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