On 03/30/20 at 09:40am, Konrad Rzeszutek Wilk wrote: > On Mon, Mar 30, 2020 at 02:06:01PM +0800, Kairui Song wrote: > > On Sat, Mar 28, 2020 at 7:57 PM Dave Young <dyoung@xxxxxxxxxx> wrote: > > > > > > On 03/26/20 at 05:29pm, 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. > > > > > > Hmm, we have a use case for kdump, this maybe useful. For example > > > swiotlb is enabled by default if AMD SME/SEV is active, and in kdump > > > kernel we have to increase the crashkernel reserved size for the extra > > > swiotlb requirement. I wonder if we can just reuse the old kernel's > > > swiotlb region and pass the addr to kdump kernel. > > > > > > > Yes, definitely helpful for kdump kernel. This can help reduce the > > crashkernel value. > > > > Previously I was thinking about something similar, play around the > > e820 entry passed to kdump and let it place swiotlb in wanted region. > > Simply remap it like in this patch looks much cleaner. > > > > If this patch is acceptable, one more patch is needed to expose the > > swiotlb in iomem, so kexec-tools can pass the right kernel cmdline to > > second kernel. > > We seem to be passsing a lot of data to kexec.. Perhaps something > of a unified way since we seem to have a lot of things to pass - disabling > IOMMU, ACPI RSDT address, and then this. acpi_rsdp kernel cmdline is not useful anymore. The initial purpose is for kexec-rebooting in efi system. But now we have supported efi boot across kexec reboot thus that is useless now. Thanks Dave
