linux-kernel - Re: 64 bit PCI access using MMX register -- how?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <adahc4as3ai.fsf@cisco.com>
Date:	Wed, 07 Jan 2009 22:46:45 -0800
From:	Roland Dreier <rdreier@...co.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Om <om.turyx@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: 64 bit PCI access using MMX register -- how?

 > I think he was ok because he saved the MMX state by itself, except:
 > 
 > - There was no guarantee that the FPU is in MMX state, not x87 state
 > - He'll often get a lazy fpu save exception. This used to BUG() 
 > in some cases when invoked from kernel space (but that might have been
 > changed now). Better is to disable this explicitely around 
 > the access (like in kernel_fpu_begin()/end())
 > - Doing this all properly is fairly expensive and I suspect
 > just using a lock will be cheaper.

I had some code a long time ago that used SSE (I think movlps was the
opcode I chose) to get an atomic 64-bit PIO operation.  To do that, I
just needed to disable preemption and save/restore cr0 around the SSE
operation, and just save/restore the single xmm register I used.  Of
course it only works on CPUs that have SSE.  That avoids the nastiness
of x87/mmx state, but in the end a spinlock around two readl()s was
faster and a ton simpler, so I threw all that code away.

 - R.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/