Date: Tue, 14 Jul 2015 11:30:23 -0700 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: Jason Harmening <jason.harmening@gmail.com> Subject: Re: RFC: New KPI for fast temporary single-page KVA mappings Message-ID: <6021359.Zicubn765k@ralph.baldwin.cx> In-Reply-To: <CAM=8qanB11WEWHZZfxyOT7VeL%2BOLqZ47bg=1TKp5c-W=VHNZnw@mail.gmail.com> References: <CAM=8qanB11WEWHZZfxyOT7VeL%2BOLqZ47bg=1TKp5c-W=VHNZnw@mail.gmail.com>
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On Tuesday, July 07, 2015 11:37:55 AM Jason Harmening wrote: > Hi everyone, > > I'd like to propose a couple of new pmap functions: > vm_offset_t pmap_quick_enter_page(vm_page_t m) > void pmap_quick_remove_page(vm_offset_t kva) > > These functions will create and destroy a temporary, usually CPU-local > mapping of the specified page. Where available, they will use the direct > map. Otherwise, they will use a per-CPU pageframe that's allocated at boot. > > Guarantees: > --Will not sleep > --Will not fail > --Safe to call under a non-spin lock or from an ithread > > Restrictions: > --Not safe to call from interrupt filter or under a spin mutex on all arches > --Mappings should be held for as little time as possible; don't do any > locking or sleeping while holding a mapping > --Current implementation only guarantees a single page of mapping space > across all arches. MI code should not make nested calls to > pmap_quick_enter_page(). > > My idea is that the first consumer of this would be busdma. All non-iommu > implementations would use this for bounce buffer copies of pages that don't > have resident mappings. Currently busdma uses physcopy[in|out] for > unmapped buffers, which on most arches uses sf_bufs that can sleep, making > bus_dmamap_sync() unsafe to call in a lot of cases. busdma would also use > this for virtually-indexed cache maintenance on arm and mips. It currently > ignores cache maintenance for buffers that don't have a KVA or resident UVA > mapping, which may not be correct for buffers that don't belong to curproc > or have cache-resident VAs on other cores. > > I've created 2 Differential reviews: > https://reviews.freebsd.org/D3013: the implementation > https://reviews.freebsd.org/D3014: the kmod I've been using to test it > > I'd like any and all feedback, both on the general approach and the > implementation details. Some things to note on the implementation: > --I've intentionally avoided touching existing pmap code for the time > being. Some of the new code could likely be shared with other pmap KPIs in > a lot of cases. > --I've structured the KPI to make it easy to extend to guarantee more than > one per-CPU page in the future. I could see that being useful for copying > between pages, for example > --There's no immediate consumer for the sparc64 implementation, since > busdma there needs neither bounce buffers nor cache maintenance. > --I would very much like feedback and testing from experts on non-x86 > arches. I only have hardware to test the i386 and amd64 implementations; > I've only cross-compiled it for everything else. Some of the non-x86 > details, like the Book E powerpc TLB invalidation code, are a bit scary and > probably not quite right. I do think something like this would be useful. What I had wanted to do was to add a 'memory cursor' to go along with memory descriptors. The idea would be that you can use a cursor to iterate over any descriptor, and that one of the options when creating a virtual address cursor was to ask it to preallocate any resources it needs at creation time (e.g. a page of KVA on platforms without a direct map). Then if a driver or GEOM module needs to walk over arbitrary I/O buffers that come down via virtual addresses, it could allocate one or more cursors. I have a partial implementation of cursors in a p4 branch, but it of course is missing the hard part of VA mappings without a direct map. However, this would let you have N of these things and to also control the lifecycle of the temporary KVA addresses instead of having a fixed set. -- John Baldwin
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