On Tuesday, December 27, 2016 09:50:32 PM Adrian Chadd wrote: > hiya, > > so I dug into the mips24k definition of this. It says this: > > " > 3.4.3 Uncached accelerated writes > The 24K core permits memory regions to be marked as “uncached > accelerated”. This type of region is useful to hard- > ware which is “write only” - perhaps video frame buffers, or some > other hardware stream. Sequential word stores in > such regions are gathered into cache-line-sized chunks, before being > written with a single burst cycle on the CPU > interface. > Such regions are uncached for read, and partial-word or > out-of-sequence writes have “unpredictable” effects - don’t > do them. The burst write is normally performed when software writes to > the last location in the memory block or does > an uncached-accelerated write to some other block; but it can also be > triggered by a > sync instruction, a pref nudge, a matching load or any exception. If > the block is not completely written by the time it’s pushed out, it > will be written using a series of doubleword or smaller write cycles > over the 24K core’s 64-bit memory interface. > " > > So, question is - is our write combining page attribute in the VM > suitable for this? Is it defined as "only do full sequential word > writes"? Or do we risk having some other platform use it in a less > "don't do this" way and then MIPS is the one at fault again? :)
The description of WC in the Intel SDM appears to be more forgiving. If you do out-of-sequence writes it won't "lose" the data, but there isn't a strong ordering of when the different regions of a cache line are finally written. Also, different WC store buffers may be written out-of-order (e.g. if you partially fill one line, then fully fill a second line, the second line may be written out, but the first one will be held for a bit to see if it will be fully filled). However, you never lose the data, and the last thing written will eventually end up being written to the bus. -- John Baldwin _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
