On Tue, Jun 22, 2021 at 1:02 PM Rob Herring <robh@xxxxxxxxxx> wrote: > > On Tue, Jun 22, 2021 at 09:06:02AM -0700, Doug Anderson wrote: > > Hi, > > > > On Tue, Jun 22, 2021 at 4:35 AM Robin Murphy <robin.murphy@xxxxxxx> wrote: > > > > > > Hi Doug, > > > > > > On 2021-06-22 00:52, Douglas Anderson wrote: > > > > > > > > This patch attempts to put forward a proposal for enabling non-strict > > > > DMA on a device-by-device basis. The patch series requests non-strict > > > > DMA for the Qualcomm SDHCI controller as a first device to enable, > > > > getting a nice bump in performance with what's believed to be a very > > > > small drop in security / safety (see the patch for the full argument). > > > > > > > > As part of this patch series I am end up slightly cleaning up some of > > > > the interactions between the PCI subsystem and the IOMMU subsystem but > > > > I don't go all the way to fully remove all the tentacles. Specifically > > > > this patch series only concerns itself with a single aspect: strict > > > > vs. non-strict mode for the IOMMU. I'm hoping that this will be easier > > > > to talk about / reason about for more subsystems compared to overall > > > > deciding what it means for a device to be "external" or "untrusted". > > > > > > > > If something like this patch series ends up being landable, it will > > > > undoubtedly need coordination between many maintainers to land. I > > > > believe it's fully bisectable but later patches in the series > > > > definitely depend on earlier ones. Sorry for the long CC list. :( > > > > > > Unfortunately, this doesn't work. In normal operation, the default > > > domains should be established long before individual drivers are even > > > loaded (if they are modules), let alone anywhere near probing. The fact > > > that iommu_probe_device() sometimes gets called far too late off the > > > back of driver probe is an unfortunate artefact of the original > > > probe-deferral scheme, and causes other problems like potentially > > > malformed groups - I've been forming a plan to fix that for a while now, > > > so I for one really can't condone anything trying to rely on it. > > > Non-deterministic behaviour based on driver probe order for multi-device > > > groups is part of the existing problem, and your proposal seems equally > > > vulnerable to that too. > > > > Doh! :( I definitely can't say I understand the iommu subsystem > > amazingly well. It was working for me, but I could believe that I was > > somehow violating a rule somewhere. > > > > I'm having a bit of a hard time understanding where the problem is > > though. Is there any chance that you missed the part of my series > > where I introduced a "pre_probe" step? Specifically, I see this: > > > > * really_probe() is called w/ a driver and a device. > > * -> calls dev->bus->dma_configure() w/ a "struct device *" > > * -> eventually calls iommu_probe_device() w/ the device. > > * -> calls iommu_alloc_default_domain() w/ the device > > * -> calls iommu_group_alloc_default_domain() > > * -> always allocates a new domain > > > > ...so we always have a "struct device" when a domain is allocated if > > that domain is going to be associated with a device. > > > > I will agree that iommu_probe_device() is called before the driver > > probe, but unless I missed something it's after the device driver is > > loaded. ...and assuming something like patch #1 in this series looks > > OK then iommu_probe_device() will be called after "pre_probe". > > > > So assuming I'm not missing something, I'm not actually relying the > > IOMMU getting init off the back of driver probe. > > > > > > > FWIW we already have a go-faster knob for people who want to tweak the > > > security/performance compromise for specific devices, namely the sysfs > > > interface for changing a group's domain type before binding the relevant > > > driver(s). Is that something you could use in your application, say from > > > an initramfs script? > > > > We've never had an initramfs script in Chrome OS. I don't know all the > > history of why (I'm trying to check), but I'm nearly certain it was a > > conscious decision. Probably it has to do with the fact that we're not > > trying to build a generic distribution where a single boot source can > > boot a huge variety of hardware. We generally have one kernel for a > > class of devices. I believe avoiding the initramfs just keeps things > > simpler. > > > > I think trying to revamp Chrome OS to switch to an initramfs type > > system would be a pretty big undertaking since (as I understand it) > > you can't just run a little command and then return to the normal boot > > flow. Once you switch to initramfs you're committing to finding / > > setting up the rootfs yourself and on Chrome OS I believe that means a > > whole bunch of dm-verity work. > > > > > > ...so probably the initramfs is a no-go for me, but I'm still crossing > > my fingers that the pre_probe() might be legit if you take a second > > look at it? > > Couldn't you have a driver flag that has the same effect as twiddling > sysfs? At the being of probe, check the flag and go set the underlying > sysfs setting in the device. My understanding of what Robin is saying is that we'd need this info well before the driver is even available. The pre_probe() is effectively doing the same thing you are suggesting. > Though you may want this to be per device, not per driver. To do that > early, I think you'd need a DT property. I wouldn't be totally opposed > to that and I appreciate you not starting there. :) Which is what I'm suggest elsewhere in the thread: https://lore.kernel.org/lkml/CAGETcx83qCZF5JN5cqXxdSFiEgfc4jYESJg-RepL2wJXJv0Eww@xxxxxxxxxxxxxx/ -Saravana
