Messages in this thread | | | From | "Michael S. Zick" <> | Subject | Re: LOCK prefix on uni processor has its use (was Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic) | Date | Wed, 27 May 2009 12:10:29 -0500 |
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On Wed May 27 2009, Harald Welte wrote: > 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. >
I am also testing with the pci cache line size hard-coded to be the same size as the processor cache line size (a WAFG for now) - -
It is too soon (only an 1 1/2 hours) to be a significant finding - - but if this was set to twice the physical line length, it would be only flushing every other line - which I think would show up *real* fast. ;)
I am noticing some "dropped buffers and/or dropped packets" in my streaming music - - but that is not conclusive of anything other than hd-audio may be using the wrong cache stride also. ;)
Mike > 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,
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