Messages in this thread | | | From | Paul Mackerras <> | Date | Tue, 3 Jul 2001 22:47:20 +1000 (EST) | Subject | Re: about kmap_high function |
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Stephen C. Tweedie writes:
> kmap_high is intended to be called routinely for access to highmem > pages. It is coded to be as fast as possible as a result. TLB > flushes are expensive, especially on SMP, so kmap_high tries hard to > avoid unnecessary flushes.
The code assumes that flushing a single TLB entry is expensive on SMP, while flushing the whole TLB is relatively cheap - certainly cheaper than flushing several individual entries. And that assumption is of course true on i386.
On PPC it is a bit different. Flushing a single TLB entry is relatively cheap - the hardware broadcasts the TLB invalidation on the bus (in most implementations) so there are no cross-calls required. But flushing the whole TLB is expensive because we (strictly speaking) have to flush the whole of the MMU hash table as well.
The MMU gets its PTEs from a hash table (which can be very large) and we use the hash table as a kind of level-2 cache of PTEs, which means that the flush_tlb_* routines have to flush entries from the MMU hash table as well. The hash table can store PTEs from many contexts, so it can have a lot of PTEs in it at any given time. So flushing the whole TLB would imply going through every single entry in the hash table and clearing it. In fact, currently we cheat - flush_tlb_all actually only flushes the kernel portion of the address space, which is all that is required in the three places where flush_tlb_all is called at the moment.
This is not a criticism, rather a request that we expand the interfaces so that the architecture-specific code can make the decisions about when and how to flush TLB entries.
For example, I would like to get rid of flush_tlb_all and define a flush_tlb_kernel_range instead. In all the places where flush_tlb_all is currently used, we do actually know the range of addresses which are affected, and having that information would let us do things a lot more efficiently on PPC. On other platforms we could define flush_tlb_kernel_range to just flush the whole TLB, or whatever.
Note that there is already a flush_tlb_range which could be used, but some architectures assume that it is only used on user addresses.
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