From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jonathan Lemon Date: Tue, 02 Jul 2019 09:33:52 -0700 Subject: [Intel-wired-lan] [PATCH 00/11] XDP unaligned chunk placement support In-Reply-To: <59AF69C657FD0841A61C55336867B5B07ED8B210@IRSMSX103.ger.corp.intel.com> References: <20190620083924.1996-1-kevin.laatz@intel.com> <20190627142534.4f4b8995@cakuba.netronome.com> <07e404eb-f712-b15a-4884-315aff3f7c7d@intel.com> <20190701142002.1b17cc0b@cakuba.netronome.com> <59AF69C657FD0841A61C55336867B5B07ED8B210@IRSMSX103.ger.corp.intel.com> Message-ID: <3510BE85-6B1B-4BB4-9640-ECEE2572DB4E@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: intel-wired-lan@osuosl.org List-ID: On 2 Jul 2019, at 2:27, Richardson, Bruce wrote: >> -----Original Message----- >> From: Jakub Kicinski [mailto:jakub.kicinski at netronome.com] >> Sent: Monday, July 1, 2019 10:20 PM >> To: Laatz, Kevin >> Cc: Jonathan Lemon ; >> netdev at vger.kernel.org; >> ast at kernel.org; daniel at iogearbox.net; Topel, Bjorn >> ; Karlsson, Magnus >> ; >> bpf at vger.kernel.org; intel-wired-lan at lists.osuosl.org; Richardson, >> Bruce >> ; Loftus, Ciara >> Subject: Re: [PATCH 00/11] XDP unaligned chunk placement support >> >> On Mon, 1 Jul 2019 15:44:29 +0100, Laatz, Kevin wrote: >>> On 28/06/2019 21:29, Jonathan Lemon wrote: >>>> On 28 Jun 2019, at 9:19, Laatz, Kevin wrote: >>>>> On 27/06/2019 22:25, Jakub Kicinski wrote: >>>>>> I think that's very limiting.? What is the challenge in >>>>>> providing >>>>>> aligned addresses, exactly? >>>>> The challenges are two-fold: >>>>> 1) it prevents using arbitrary buffer sizes, which will be an >>>>> issue >>>>> supporting e.g. jumbo frames in future. >>>>> 2) higher level user-space frameworks which may want to use >>>>> AF_XDP, >>>>> such as DPDK, do not currently support having buffers with 'fixed' >>>>> alignment. >>>>> ??? The reason that DPDK uses arbitrary placement is that: >>>>> ??? ??? - it would stop things working on certain NICs which >>>>> need >>>>> the actual writable space specified in units of 1k - therefore we >>>>> need 2k >>>>> + metadata space. >>>>> ??? ??? - we place padding between buffers to avoid >>>>> constantly >>>>> hitting the same memory channels when accessing memory. >>>>> ??? ??? - it allows the application to choose the actual >>>>> buffer >>>>> size it wants to use. >>>>> ??? We make use of the above to allow us to speed up processing >>>>> significantly and also reduce the packet buffer memory size. >>>>> >>>>> ??? Not having arbitrary buffer alignment also means an AF_XDP >>>>> driver for DPDK cannot be a drop-in replacement for existing >>>>> drivers in those frameworks. Even with a new capability to allow >>>>> an >>>>> arbitrary buffer alignment, existing apps will need to be modified >>>>> to use that new capability. >>>> >>>> Since all buffers in the umem are the same chunk size, the original >>>> buffer address can be recalculated with some multiply/shift math. >>>> However, this is more expensive than just a mask operation. >>> >>> Yes, we can do this. >> >> That'd be best, can DPDK reasonably guarantee the slicing is uniform? >> E.g. it's not desperate buffer pools with different bases? > > It's generally uniform, but handling the crossing of (huge)page > boundaries > complicates things a bit. Therefore I think the final option below > is best as it avoids any such problems. > >> >>> Another option we have is to add a socket option for querying the >>> metadata length from the driver (assuming it doesn't vary per >>> packet). >>> We can use that information to get back to the original address >>> using >>> subtraction. >> >> Unfortunately the metadata depends on the packet and how much info >> the >> device was able to extract. So it's variable length. >> >>> Alternatively, we can change the Rx descriptor format to include the >>> metadata length. We could do this in a couple of ways, for example, >>> rather than returning the address as the start of the packet, >>> instead >>> return the buffer address that was passed in, and adding another >>> 16-bit field to specify the start of packet offset with that buffer. >>> If using another 16-bits of the descriptor space is not desirable, >>> an >>> alternative could be to limit umem sizes to e.g. 2^48 bits (256 >>> terabytes should be enough, right :-) ) and use the remaining 16 >>> bits >>> of the address as a packet offset. Other variations on these >>> approach >>> are obviously possible too. >> >> Seems reasonable to me.. > > I think this is probably the best solution, and also has the advantage > that > a buffer retains its base address the full way through the cycle of Rx > and Tx. I like this as well - it also has the advantage that drivers can keep performing adjustments on the handle, which ends up just modifying the offset. -- Jonathan