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Re: Anyone noticed that there are a lot of cache flushes after kunmap/ku

To: David VomLehn <dvomlehn@cisco.com>
Subject: Re: Anyone noticed that there are a lot of cache flushes after kunmap/kunmap_atomic is called?
From: "Kevin D. Kissell" <kevink@paralogos.com>
Date: Tue, 12 Aug 2008 11:57:43 +0200
Cc: "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>
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I remember dealing with analgous situations in System V many many years ago. If the cache flushes are virtual, then either there must be bullet-proof protection (hardware or software) against virtual aliasing, or they must be flushed on deallocation, before the mapping is destroyed. The good news is that, provided that DMA buffer management is done correctly, one should be able to use hit-invalidate instead of hit-writeback-invalidate on the deallocated pages. That won't save that many CPU cycles, but it will save some number of memory cycles and milliwatts.

         Regards,

         Kevin K.

David VomLehn wrote:
On the MIPS processor, cache flushing is done based on virtual addresses. However, in the Linux kernel, there are a lot of places where memory is mapped with kmap or kmap_atomic, then unmapped with the corresponding kunmap or kunmap_atomic and only *then* is the cache flushed. In other words, we only flush the cache after we have dropped the mapping of memory into a virtual address. I think this is generally wrong.

This may really only affect those of us who have enabled high memory, but it's pretty prevalent in kernel code. We noted this before, but have apparently just been bitten by it. Is it just me or is there a fairly widespread problem for processors that flush the cache using virtual addresses?




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