"H. Peter Anvin" <hpa at zytor.com> writes: > On 08/11/2010 12:47 PM, Neil Horman wrote: >> Hey all- >> I've got a question regarding x86_64 and how linux uses the paging >> hardware. I'm tinkering with ways to get kexec to boot a new kernel on panic >> without leaving long mode. The idea being that if we can do that, then we don't >> need to store the new kdump kernel below the 4G physical limit for 32 bit >> systems. In doing this though, I figured I would have to re-initalize the page >> table with an identity mapped set of page tables to cover all of ram and load >> that into cr3. My question is, is it safe to do so while paging is enabled. >> The docs I've read are unclear on that and if I have to disable paging that >> automatically drops me out of long mode, which is bad. I would think its safe >> to do, since I imagined we had to do on context switches in the scheduler, but >> the __switch_to implementation for x86_64 sems to do nothing but update the task >> register. Intel vol 3a says we need to update cr3, but I don't see where that >> happens, so I'm not sure if theres some automated bit that does a cr3 update >> safely when we write tr. >> >> Anywho, any guidance, clarification would be appreciated. Thanks! >> Neil >> > > It is definitely safe to load a new CR3 while paging is done; it is done > all the time. The currently executing page needs to be mapped to the > same physical and virtual address in most kernels. > > However, there are a *LOT* of issues with having a kernel that is > completely above 4 GiB. For one thing, a lot of device drivers simply > will not work if there is no memory below 4 GiB awavilable to the > kernel. As such, I don't think you will be successful in this > project. A couple of pieces. 1) The kernel side of kexec and kexec on panic does not leave long mode. Long mode is left by the glue code in /sbin/kexec. 2) I agree about the DMA limitation however there are enough systems with iommu's these days you may be able to get it to work. 3) I would start just getting the normal kexec case to work. The 64bit kernel does support starting at the 64bit entry point, but I don't think it has been tested if loaded above 4G. It certainly should work and as time goes by I expect running a kernel above 4G to become an increasingly interesting use case. So it is certainly worth play with. But as Peter says having a kernel completely above 4GiB has is likely to uncover a lot of baked in assumptions so we real problems might result. Hmm. On the normal kexec side you don't loose the low 4GiB so that case should be a lot easier to bootstrap with. Once it works with the low 4GiB you can add a mem= or whatever to disable using the low 4GiB and see what happens. Have fun. Eric
