On Mon, 12 Oct 2020 at 18:22, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Mon, Oct 12, 2020 at 05:55:45PM +0200, Ard Biesheuvel wrote: > > On Mon, 12 Oct 2020 at 17:50, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > > > > On Mon, Oct 12, 2020 at 12:43:05PM +0200, Ard Biesheuvel wrote: > > > > > > Also, could someone give an executive summary of why it matters where > > > > > > the crashkernel is loaded? As far as I can tell, reserve_crashkernel() > > > > > > only allocates memory for the kernel's executable image itself, which > > > > > > can usually be loaded anywhere in memory. I could see how a > > > > > > crashkernel might need some DMA'able memory if it needs to use the > > > > > > hardware, but I don't think that is what is going on here. > [...] > > > However, the crashkernel=... range is meant for sufficiently large > > > reservation to be able to run the kdump kernel, not just load the image. > > > > Sure. But I was referring to the requirement that it is loaded low in > > memory. Unless I am misunderstanding something, all we need for the > > crashkernel to be able to operate is some ZONE_DMA memory in case it > > is needed by the hardware, and beyond that, it could happily live > > anywhere in memory. > > Yes, the crash kernel doesn't need to be loaded in the low memory. But > some low memory needs to end up in its perceived System RAM. That's what > Chen is trying to do with this series: > > https://lore.kernel.org/linux-arm-kernel/20200907134745.25732-1-chenzhou10@xxxxxxxxxx/ > > It reserves the normal crashkernel memory at some high address range > with a small block (currently proposed as 256MB similar to x86) in the > "low" range. > > This "low" range for arm64 currently means below 1GB but it's only RPi4 > that needs it this low, all other platforms are fine with the full low > 32-bit range. > > If it's not doable in a nice way, we'll just leave with this permanent > 1GB ZONE_DMA and hope we won't get platforms requiring an even smaller > one. There's also the option of ignoring kdump on RPi4, make ZONE_DMA > depend on !CRASH_DUMP and the "low" reservations can use the full 32-bit > range since the kdump kernel won't need <30-bit addresses. > Are you aware of any reason why we cannot defer the call to reserve_crashkernel() to the start of bootmem_init()? That way, we have access to the unflattened DT as well as the IORT, and so we can tweak the zone limits based on the h/w description, but before allocating the crashkernel.
