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Date: Wed, 25 Mar 2015 10:28:45 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>,
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Subject: Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss
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* Andy Lutomirski <luto@amacapital.net> wrote:

> Now we can do a fun hack on top.  On Intel, we have 
> sysenter/sysexitl and, on AMD, we have syscall/sysretl.  But, if I 
> read the docs right, Intel has sysretl, too.  So we can ditch 
> sysexit entirely, since this mechanism no longer has any need to 
> keep the entry and exit conventions matching.

So this only affects 32-bit vdsos, because on 64-bit both Intel and 
AMD have and use SYSCALL/SYSRET.

So my question would be: what's the performance difference between 
INT80 and sysenter entries on 32-bit, on modern CPUs?

If it's not too horrible (say below 100 cycles) then we could say that 
we start out the simplification and robustification by switching Intel 
over to INT80 + SYSRET on 32-bit, and once we know the 32-bit SYSRET 
and all the other simplifications work fine we implement the 
SYSENTER-hack on top of that?

Is there any user-space code that relies on being able to execute an 
open coded SYSENTER, or are we shielded via the vDSO?

Doing it this way would make it a lot more practical to pull off, as 
the incentive to implement the SYSENTER hack on Intel CPUs will be 
significant: dozens of cycles on 32-bit. Also, I have no problem with 
putting some pressure on Intel developers, for the absolutely 
indefensible horror interface that SYSENTER turned out to be! ;-)

Thanks,

	Ingo