On Thu, Mar 26, 2020 at 06:11:31PM +0100, Alexander Graf wrote: > On 26.03.20 18:05, Christoph Hellwig wrote: > > > > On Thu, Mar 26, 2020 at 05:29:22PM +0100, Alexander Graf wrote: > > > The swiotlb is a very convenient fallback mechanism for bounce buffering of > > > DMAable data. It is usually used for the compatibility case where devices > > > can only DMA to a "low region". > > > > > > However, in some scenarios this "low region" may be bound even more > > > heavily. For example, there are embedded system where only an SRAM region > > > is shared between device and CPU. There are also heterogeneous computing > > > scenarios where only a subset of RAM is cache coherent between the > > > components of the system. There are partitioning hypervisors, where > > > a "control VM" that implements device emulation has limited view into a > > > partition's memory for DMA capabilities due to safety concerns. > > > > > > This patch adds a command line driven mechanism to move all DMA memory into > > > a predefined shared memory region which may or may not be part of the > > > physical address layout of the Operating System. > > > > > > Ideally, the typical path to set this configuration would be through Device > > > Tree or ACPI, but neither of the two mechanisms is standardized yet. Also, > > > in the x86 MicroVM use case, we have neither ACPI nor Device Tree, but > > > instead configure the system purely through kernel command line options. > > > > > > I'm sure other people will find the functionality useful going forward > > > though and extend it to be triggered by DT/ACPI in the future. > > > > I'm totally against hacking in a kernel parameter for this. We'll need > > a proper documented DT or ACPI way. > > I'm with you on that sentiment, but in the environment I'm currently looking > at, we have neither DT nor ACPI: The kernel gets purely configured via > kernel command line. For other unenumerable artifacts on the system, such as > virtio-mmio platform devices, that works well enough and also basically > "hacks a kernel parameter" to specify the system layout. On the arm64 front, you'd *have* to pass a DT to the kernel (as that's where we get the command line from), and we *only* discover memory from the DT or EFI memory map, so the arguments above aren't generally applicable. You can enumerate virtio-mmio devices from DT, also. Device-specific constraints on memory should really be described in a per-device fashion in the FW tables so that the OS can decide how to handle them. Just becuase one device can only access memory in a specific 1MiB window doesn't mean all other should be forced to share the same constraint. I think that's what Christoph was alluding to. Thanks, Mark.
