On Wed, 19 May 2021 09:53:33 -0700 Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > On Wed, May 19, 2021 at 9:35 AM Jonathan Cameron > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > On Wed, 19 May 2021 17:20:52 +0100 > > Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > > On Wed, 19 May 2021 08:29:58 -0700 > > > Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > > > > > On Wed, May 19, 2021 at 8:14 AM Jonathan Cameron > > > > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > > > > > > > On Wed, 19 May 2021 07:18:28 -0700 > > > > > Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > > > > > > > > > On Tue, May 18, 2021 at 3:06 AM Jonathan Cameron > > > > > > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > On Mon, 17 May 2021 10:21:14 -0700 > > > > > > > Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > > > > > > > > > > > > > On Mon, May 17, 2021 at 1:42 AM Jonathan Cameron > > > > > > > > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > On Fri, 14 May 2021 11:37:12 -0700 > > > > > > > > > Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > > On Fri, May 14, 2021 at 1:50 AM Jonathan Cameron > > > > > > > > > > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > > > > > > > [..] > > > > > > > > > > > > If it simplifies the kernel implementation to assume single > > > > > > > > > > > > kernel-initiator then I think that's more than enough reason to block > > > > > > > > > > > > out userspace, and/or provide userspace a method to get into the > > > > > > > > > > > > kernel's queue for service. > > > > > > > > > > > > > > > > > > > > > > This last suggestion makes sense to me. Let's provide a 'right' way > > > > > > > > > > > to access the DOE from user space. I like the idea if it being possible > > > > > > > > > > > to run CXL compliance tests from userspace whilst the driver is loaded. > > > > > > > > > > > > > > > > > > > > Ah, and I like your observation that once the kernel provides a > > > > > > > > > > "right" way to access DOE then userspace direct-access of DOE is > > > > > > > > > > indeed a "you get to keep the pieces" event like any other unwanted > > > > > > > > > > userspace config-write. > > > > > > > > > > > > > > > > > > > > > Bjorn, given this would be a generic PCI thing, any preference for what > > > > > > > > > > > this interface might look like? /dev/pcidoe[xxxxxx].i with ioctls similar > > > > > > > > > > > to those for the BAR based CXL mailboxes? > > > > > > > > > > > > > > > > > > > > (warning, anti-ioctl bias incoming...) > > > > > > > > > > > > > > > > > > I feel very similar about ioctls - my immediate thought was to shove this in > > > > > > > > > debugfs, but that feels the wrong choice if we are trying to persuade people > > > > > > > > > to use it instead of writing code that directly accesses the config space. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hmm, DOE has an enumeration capability, could the DOE driver use a > > > > > > > > > > scheme to have a sysfs bin_attr per discovered object type? This would > > > > > > > > > > make it simliar to the pci-vpd sysfs interface. > > > > > > > > > > > > > > > > > > We can discover the protocols, but anything beyond that is protocol > > > > > > > > > specific. I don't think there is a enough info available by any standards > > > > > > > > > defined method. Also part of the reason to allow a safe userspace interface > > > > > > > > > would be to provide a generic interface for vendor protocols and things like > > > > > > > > > CXL compliance tests where we will almost certainly never provide a more > > > > > > > > > specific kernel interface. > > > > > > > > > > > > > > > > > > Whilst sysfs would work for CDAT, some protocols are challenge response rather > > > > > > > > > than simple read back and that really doesn't fit well for sysfs model. > > > > > > > > > If we get other protocols that are simple data read back, then I would > > > > > > > > > advocate giving them a simple sysfs interface much like proposed for CDAT > > > > > > > > > as it will always be simpler to use + self describing. > > > > > > > > > > > > > > > > > > On a lesser note it might be helpful to provide sysfs attrs for > > > > > > > > > what protocols are supported. The alternative is to let userspace run > > > > > > > > > the discovery protocol. Perhaps we can do this as a later phase. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Then the kernel could cache objects like CDAT that don't change > > > > > > > > > > outside of some invalidation event. > > > > > > > > > > > > > > > > > > It's been a while since I last saw any conversation on sysfs bin_attrs > > > > > > > > > but mostly I thought the feeling was pretty strongly against them for anything > > > > > > > > > but a few niche usecases. > > > > > > > > > > > > > > > > > > Feels to me like it would break most of the usual rules in a way vpd does > > > > > > > > > not (IIRC VPD is supposed to be a simple in the sense that if you write a value > > > > > > > > > to a writable part, you will read back the same value). > > > > > > > > > > > > > > > > > > +CC Greg who is a fount of knowledge in this area (and regularly + correctly > > > > > > > > > screams at the ways I try to abuse sysfs :) Note I don't think Dan was > > > > > > > > > suggesting implementing response / request directly, but I think that is > > > > > > > > > all we could do given DOE protocols can be vendor specific and the standard > > > > > > > > > discovery protocol doesn't let us know the fine grained support (what commands > > > > > > > > > within a given protocol). > > > > > > > > > > > > > > > > I'm not all that interested in supporting vendor defined DOE > > > > > > > > shenanigans. There's more than enough published DOE protocols that the > > > > > > > > kernel could limit its support to the known set. This is similar to > > > > > > > > how ACPI DSMs are not generically supported, but when they appear in a > > > > > > > > published specification the kernel may then grow the support. The > > > > > > > > supported protocols could be limited to: CDAT, PCIe IDE, CXL > > > > > > > > Compliance, etc... > > > > > > > > > > > > > > > > Vendor specific DOE is in the same class as unfettered /dev/mem > > > > > > > > access, first you need to disable the kernel's integrity and > > > > > > > > confidentiality protections, and then you can do whatever you want. If > > > > > > > > a vendor wants a DOE protocol supported in the "trusted" set they can > > > > > > > > simply publish the specification and send the proper support patches. > > > > > > > > > > > > > > Fair enough, though the interface should be root only, so a vendor shooting > > > > > > > themselves in the foot this way would be no different to using pcitools > > > > > > > to access the device directly (we are just providing safety from concurrency > > > > > > > point of view). > > > > > > > > > > > > > > Anyway, I can see two options for how to do this. > > > > > > > > > > > > > > 1) Per protocol interface. Would not be generic, as these work in entirely > > > > > > > different ways (some are simple read back of tables, some require complex > > > > > > > cycles of operations in the right order with data flowing in both directions) > > > > > > > 2) White list those protocols we are going to let through a generic interface > > > > > > > Not including CXL compliance for instance as that has nasty side effects! > > > > > > > > > > > > > > If we want to enable userspace DOE access, I prefer option 2. > > > > > > > > > > > > > > Note that I wasn't that keen on a userspace interface in the first place as > > > > > > > in my view these should all be handled in kernel. > > > > > > > Ultimately we should have case 1 if userspace access make sense. > > > > > > > However, if we do this we shouldn't pretend we are providing userspace > > > > > > > access to the DOE at all. We are providing interfaces to things that just > > > > > > > happen to be implemented using DOE under the hood. > > > > > > > > > > > > > > I have a prototype of a trivial ioctl based interface. I'll send it out > > > > > > > as an RFC later this week. Might add a white list, depending on where > > > > > > > this discussion goes. > > > > > > > > > > > > > > > > > > > I'd say let's do this in typical Linux fashion and not solve future > > > > > > problems before they need to be solved. I.e. start small and build > > > > > > incrementally. To me that looks like a sysfs interface to convey a > > > > > > cached copy of a CDAT with an internal interface for a driver to > > > > > > trigger invalidations and re-reads on the next access. This would > > > > > > assume that userspace may have left the DOE in an indeterminate state > > > > > > and an abort cycle may be needed. A 1 second delay for the rare case > > > > > > where a collision is detected seems reasonable for just CDAT > > > > > > retrieval. > > > > > > > > > > The problem is you can not detect a collision. > > > > > > > > This discussion started because Ira questioned the handling of the > > > > busy status. If the DOE is busy and the kernel did not make it busy > > > > then there was a collision, no? > > > > > > True, but not complete. Not having busy set does not mean there > > > wasn't a collision. Busy is an indication that the EP can't recieve > > > a new request (no space in buffer or similar), not that there is a response > > > still to be sent back. We have no way to tell if there is a response > > > going to come back in the future. There is no 'exchange in flight' flag. > > > > Perhaps useful to add a quote from the DOE ECN here. > > This is in an implementation note on Page 6. > > > > "The DOE Busy bit can be used to indicate that the DOE responder is > > temporarily unable to accept a data object. It is necessary for a > > DOE requester to ensure that individual data object transfers are > > completed, and that a request/response contract is completed, for > > example using a mutex mechanism to block other conflicting traffic > > for cases where such conflicts are possible." > > I read that as the specification mandating my proposal to disallow > multi-initiator access. My only mistake was making the exclusion apply > to reads and not limiting it to the minimum of config write exclusion. Key thing is even that isn't enough. The mutex isn't about stopping temporary access, it's about ensuring "request/response contract is completed". So you would need userspace to be able to take a lock to stop the kernel from using the DOE whilst it completes it's request/response pair and userspace to guarantee it doesn't do anything stupid. Easiest way to do that is provide proper interfaces that allows the kernel to fully mediate the access + don't support direct userspace access for normal operation. (treat it the same as an other config space write) Jonathan
