On 2019/6/6 0:32, James Morse wrote: > Hi! > > On 07/05/2019 04:50, Chen Zhou wrote: >> We use crashkernel=X to reserve crashkernel below 4G, which will fail >> when there is no enough memory. Currently, crashkernel=Y@X can be used >> to reserve crashkernel above 4G, in this case, if swiotlb or DMA buffers >> are requierd, capture kernel will boot failure because of no low memory. > >> When crashkernel is reserved above 4G in memory, kernel should reserve >> some amount of low memory for swiotlb and some DMA buffers. So there may >> be two crash kernel regions, one is below 4G, the other is above 4G. > > This is a good argument for supporting the 'crashkernel=...,low' version. > What is the 'crashkernel=...,high' version for? > > Wouldn't it be simpler to relax the ARCH_LOW_ADDRESS_LIMIT if we see 'crashkernel=...,low' > in the kernel cmdline? > > I don't see what the 'crashkernel=...,high' variant is giving us, it just complicates the > flow of reserve_crashkernel(). > > If we called reserve_crashkernel_low() at the beginning of reserve_crashkernel() we could > use crashk_low_res.end to change some limit variable from ARCH_LOW_ADDRESS_LIMIT to > memblock_end_of_DRAM(). > I think this is a simpler change that gives you what you want. According to your suggestions, we should do like this: 1. call reserve_crashkernel_low() at the beginning of reserve_crashkernel() 2. mark the low region as 'nomap' 3. use crashk_low_res.end to change some limit variable from ARCH_LOW_ADDRESS_LIMIT to memblock_end_of_DRAM() 4. rename crashk_low_res as "Crash kernel (low)" for arm64 5. add an 'linux,low-memory-range' node in DT Do i understand correctly? > > >> Then >> Crash dump kernel reads more than one crash kernel regions via a dtb >> property under node /chosen, >> linux,usable-memory-range = <BASE1 SIZE1 [BASE2 SIZE2]>. > > Won't this break if your kdump kernel doesn't know what the extra parameters are? > Or if it expects two ranges, but only gets one? These DT properties should be treated as > ABI between kernel versions, we can't really change it like this. > > I think the 'low' region is an optional-extra, that is never mapped by the first kernel. I > think the simplest thing to do is to add an 'linux,low-memory-range' that we > memblock_add() after memblock_cap_memory_range() has been called. > If its missing, or the new kernel doesn't know what its for, everything keeps working. > > >> Besides, we need to modify kexec-tools: >> arm64: support more than one crash kernel regions(see [1]) > >> I post this patch series about one month ago. The previous changes and >> discussions can be retrived from: > > Ah, this wasn't obvious as you've stopped numbering the series. Please label the next one > 'v6' so that we can describe this as 'v5'. (duplicate numbering would be even more confusing!) > ok. > > Thanks, > > James > > . > Thanks, Chen Zhou
