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
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