Question regardin intel64 arch and page table setup

[H. Peter Anvin]

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