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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/
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