Messages in this thread | | | Subject | Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address | From | Alexander Graf <> | Date | Thu, 26 Mar 2020 18:11:31 +0100 |
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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.
> We also need to feed this information > into the actual DMA bounce buffering decisions and not just the swiotlb > placement.
Care to elaborate a bit here? I was under the impression that "swiotlb=force" basically allows you to steer the DMA bounce buffering decisions already.
Thanks!
Alex
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