linux-kernel - Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32

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Message-ID: <20150325092845.GA1809@gmail.com>
Date:	Wed, 25 Mar 2015 10:28:45 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

* Andy Lutomirski <luto@...capital.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
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