linux-kernel - Re: [PATCH 2/4] x86/asm/entry: Untangle 'ia32_sysenter_target' into two entry points: entry_SYSENTER_32 and entry_SYSENTER

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Date:	Tue, 9 Jun 2015 09:33:37 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/4] x86/asm/entry: Untangle 'ia32_sysenter_target' into
 two entry points: entry_SYSENTER_32 and entry_SYSENTER_compat

On Tue, Jun 9, 2015 at 2:33 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
>> On Mon, Jun 8, 2015 at 1:34 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> > So the SYSENTER instruction is pretty quirky and it has different behavior
>> > depending on bitness and CPU maker.
>> >
>> > Yet we create a false sense of coherency by naming it 'ia32_sysenter_target'
>> > in both of the cases.
>> >
>> > Split the name into its two uses:
>> >
>> >         ia32_sysenter_target (32)    -> entry_SYSENTER_32
>> >         ia32_sysenter_target (64)    -> entry_SYSENTER_compat
>> >
>>
>> Now that I'm rebasing my pile on top of this, I have a minor gripe
>> about this one.  There are (in my mind, anyway), two SYSENTER
>> instructions: the 32-bit one and the 64-bit one.  (That is, there's
>> SYSENTER32, which happens when you do SYSENTER in 32-bit or compat
>> mode, and SYSENTER64, which happens when you do SYSENTER in long
>> mode.)  SYSENTER32, from user code's perspective, does the same thing
>> in either case [1].  That means that it really does make sense that
>> we'd have two implementations of the same entry point, one written in
>> 32-bit asm and one written in 64-bit asm.
>>
>> The patch I'm rebasing merges the two wrmsrs to MSR_IA32_SYSENTER, and
>> this change makes it uglier.
>>
>> [1] Sort of.  We probably have differently nonsensical calling
>> conventions, but that's our fault and has nothing to do with the
>> hardware.
>
> Did you intend to merge these two wrmsr()s:
>
>  #ifdef CONFIG_X86_64
>  void syscall_init(void)
>  {
>  ...
>          wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat);
>  ...
>  }
>  #endif
>
>  #ifdef CONFIG_X86_32
>  void enable_sep_cpu(void)
>  {
>  ...
>          wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long)entry_SYSENTER_32, 0);
>  ...
>  }
>
> ... and the new bifurcated names preserve the #ifdef, right?

Exactly.

>
> So I mostly agree with you, but still I'm a bit torn about this, for the following
> reason:
>
>  - SYSENTER on a 32-bit kernel behaves a bit differently from SYSENTER on a 64-bit
>    kernel: for example on 32-bit kernels we'll return with SYSEXIT, while on
>    64-bit kernels we return with SYSRET. The difference is small but user-space
>    observable: for example EDX is 0 on SYSRET while it points to ->sysenter_return
>    in the SYSEXIT case.
>
> This kind of user-observable assymmetry does not exist for other unified syscall
> ABIs, such as the INT80 method.
>
> So I think that despite having to preserve a small non-unified #ifdef for this
> initialization, we are still better off naming the two entry points differently,
> along the pattern we use, because the behavior is slightly different depending on
> the bitness of the kernel.
>

Fair enough.  This is certainly not a big deal either way.  Maybe when
this really gets cleaned up, we can merge the entry points again.

--Andy

> Thanks,
>
>         Ingo



-- 
Andy Lutomirski
AMA Capital Management, LLC
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