On 3/31/2017 6:42 PM, Logan Gunthorpe wrote: > > > On 31/03/17 03:38 PM, Sinan Kaya wrote: >> On 3/31/2017 5:23 PM, Logan Gunthorpe wrote: >>> What exactly would you white/black list? It can't be the NIC or the >>> disk. If it's going to be a white/black list on the switch or root port >>> then you'd need essentially the same code to ensure they are all behind >>> the same switch or root port. >> >> What is so special about being connected to the same switch? >> >> Why don't we allow the feature by default and blacklist by the root ports >> that don't work with a quirk. > > Well root ports have the same issue here. There may be more than one > root port or other buses (ie QPI) between the devices in question. So > you can't just say "this system has root port X therefore we can always > use p2pmem". We only care about devices on the data path between two devices. > In the end, if you want to do any kind of restrictions > you're going to have to walk the tree, as the code currently does, and > figure out what's between the devices being used and black or white list > accordingly. Then seeing there's just such a vast number of devices out > there you'd almost certainly have to use some kind of white list and not > a black list. Then the question becomes which devices will be white > listed? How about a combination of blacklist + time bomb + peer-to-peer feature? You can put a restriction with DMI/SMBIOS such that all devices from 2016 work else they belong to blacklist. > The first to be listed would be switches seeing they will always > work. This is pretty much what we have (though it doesn't currently > cover multiple levels of switches). The next step, if someone wanted to > test with specific hardware, might be to allow the case where all the > devices are behind the same root port which Intel Ivy Bridge or newer. Sorry, I'm not familiar with Intel architecture. Based on what you just wrote, I think I see your point. I'm trying to generalize what you are doing to a little bigger context so that I can use it on another architecture like arm64 where I may or may not have a switch. This text below is sort of repeating what you are writing above. How about this: The goal is to find a common parent between any two devices that need to use your code. - all bridges/switches on the data need to support peer-to-peer, otherwise stop. - Make sure that all devices on the data path are not blacklisted via your code. - If there is at least somebody blacklisted, we stop and the feature is not allowed. - If we find a common parent and no errors, you are good to go. - We don't care about devices above the common parent whether they have some feature X, Y, Z or not. Maybe, a little bit less code than what you have but it is flexible and not that too hard to implement. Well, the code is in RFC. I don't see why we can't remove some restrictions and still have your code move forward. > However, I don't think a comprehensive white list should be a > requirement for this work to go forward and I don't think anything > you've suggested will remove any of the "ugliness". I don't think the ask above is a very big deal. If you feel like addressing this on another patchset like you suggested in your cover letter, I'm fine with that too. > > What we discussed at LSF was that only allowing cases with a switch was > the simplest way to be sure any given setup would actually work. > >> I'm looking at this from portability perspective to be honest. > > I'm looking at this from the fact that there's a vast number of > topologies and devices involved, and figuring out which will work is > very complicated and could require a lot of hardware testing. The LSF > folks were primarily concerned with not having users enable the feature > and see breakage or terrible performance. > >> I'd rather see the feature enabled by default without any assumptions. >> Using it with a switch is just a use case that you happened to test. >> It can allow new architectures to use your code tomorrow. > > That's why I was advocating for letting userspace decide such that if > you're setting up a system with this you say to use a specific p2pmem > device and then you are responsible to test and benchmark it and decide > to use it in going forward. However, this has received a lot of push back. Yeah, we shouldn't trust the userspace for such things. > > Logan > -- Sinan Kaya Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
