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. -hpa
