ebiederm at xmission.com (Eric W. Biederman) writes: > "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. I guess the one place where we have a bottleneck with loading above 4GiB today is that we don't export the kernels 4GiB entry point in bzImage (although it is at a stable offset from the 32bit one), and we can't make up the kernel parameters from scratch because there are variables in there with non-zero changing values that the kernel expects to have initialized. But hacking around that for testing should not be hard. Eric
