Messages in this thread | | | Date | Wed, 27 May 2009 19:01:18 +0200 | From | Harald Welte <> | Subject | LOCK prefix on uni processor has its use (was Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic) |
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Hi hpa and others,
On Sat, May 23, 2009 at 04:44:08PM -0700, H. Peter Anvin wrote:
> It looks like there might be a problem with the C7-M ... Michael reports > that if he sets LOCK_PREFIX to "lock;" it works, but that shouldn't be > necessary for a uniprocessor.
It seems, they are neccessary.
Here are some statements from the CPU logic guys at VIA/Centaur:
* A read-modify-write sequence cannot be interupted. * All X86 instructions except rep-strings are atomic wrt interrupts. * The lock prefix has uses on a UP processor: It keeps DMA devices from interfering with a read-modify-write sequence
Furthermore, they have done some experimentation in the past, making the CPU simply ignore the LOCK prefix on uni-processor (running a certain popular proprietary operating system): It doesn't work, presumably of the abovementioned DMA related conflict.
Also, the engineers believe that it is only a matter of time until different CPU/chipset combination would expose the same bug. Since the in-order single-retire C7-M is more vulnerable than out-of-order, multiple-retire CPU's, they are not surprised that the issue shows first on the C7-M.
The recommendation from the CPU engineers, unsurprisingly, thus is to put the LOCK prefixes back where they were.
Hope this helps you.
Now if I understand the issues correctly, it would mean that there is some driver code that modifies a certain chunk of memory, while DMA of some peripheral is also accessing that memory. I suppose it would not have to be the same actual address, but probably being within the same cache line is already sufficient.
Now the question is: Is this a valid operation of a driver? Should the driver do such things, or is such a driver broken? When would that occur? I'm trying to come up with a case, but typically you e.g. allocate some DMA buffer and then don't touch it until the hardware has processed it.
Regards, -- - Harald Welte <HaraldWelte@viatech.com> http://linux.via.com.tw/ ============================================================================ VIA Free and Open Source Software Liaison
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