On Wed, Mar 13, 2019 at 09:37:08AM -0700, James Bottomley wrote: > On Wed, 2019-03-13 at 09:05 -0700, Christoph Hellwig wrote: > > On Tue, Mar 12, 2019 at 01:53:37PM -0700, James Bottomley wrote: > > > I've got to say: optimize what? What code do we ever have in the > > > kernel that kmap's a page and then doesn't do anything with it? You > > > can > > > guarantee that on kunmap the page is either referenced (needs > > > invalidating) or updated (needs flushing). The in-kernel use of > > > kmap is > > > always > > > > > > kmap > > > do something with the mapped page > > > kunmap > > > > > > In a very short interval. It seems just a simplification to make > > > kunmap do the flush if needed rather than try to have the users > > > remember. The thing which makes this really simple is that on most > > > architectures flush and invalidate is the same operation. If you > > > really want to optimize you can use the referenced and dirty bits > > > on the kmapped pte to tell you what operation to do, but if your > > > flush is your invalidate, you simply assume the data needs flushing > > > on kunmap without checking anything. > > > > I agree that this would be a good way to simplify the API. Now > > we'd just need volunteers to implement this for all architectures > > that need cache flushing and then remove the explicit flushing in > > the callers.. > > Well, it's already done on parisc ... I can help with this if we agree > it's the best way forward. It's really only architectures that > implement flush_dcache_page that would need modifying. > > It may also improve performance because some kmap/use/flush/kunmap > sequences have flush_dcache_page() instead of > flush_kernel_dcache_page() and the former is hugely expensive and > usually unnecessary because GUP already flushed all the user aliases. > > In the interests of full disclosure the reason we do it for parisc is > because our later machines have problems even with clean aliases. So > on most VIPT systems, doing kmap/read/kunmap creates a fairly harmless > clean alias. Technically it should be invalidated, because if you > remap the same page to the same colour you get cached stale data, but > in practice the data is expired from the cache long before that > happens, so the problem is almost never seen if the flush is forgotten. > Our problem is on the P9xxx processor: they have a L1/L2 VIPT L3 PIPT > cache. As the L1/L2 caches expire clean data, they place the expiring > contents into L3, but because L3 is PIPT, the stale alias suddenly > becomes the default for any read of they physical page because any > update which dirtied the cache line often gets written to main memory > and placed into the L3 as clean *before* the clean alias in L1/L2 gets > expired, so the older clean alias replaces it. > > Our only recourse is to kill all aliases with prejudice before the > kernel loses ownership. > > > > > Which means after we fix vhost to add the flush_dcache_page after > > > > kunmap, Parisc will get a double hit (but it also means Parisc > > > > was the only one of those archs needed explicit cache flushes, > > > > where vhost worked correctly so far.. so it kinds of proofs your > > > > point of giving up being the safe choice). > > > > > > What double hit? If there's no cache to flush then cache flush is > > > a no-op. It's also a highly piplineable no-op because the CPU has > > > the L1 cache within easy reach. The only event when flush takes a > > > large amount time is if we actually have dirty data to write back > > > to main memory. > > > > I've heard people complaining that on some microarchitectures even > > no-op cache flushes are relatively expensive. Don't ask me why, > > but if we can easily avoid double flushes we should do that. > > It's still not entirely free for us. Our internal cache line is around > 32 bytes (some have 16 and some have 64) but that means we need 128 > flushes for a page ... we definitely can't pipeline them all. So I > agree duplicate flush elimination would be a small improvement. > > James I suspect we'll keep the copyXuser path around for 32 bit anyway - right Jason? So we can also keep using that on parisc... -- MST
