Messages in this thread | | | Subject | Re: [Fastboot] [PATCH 03/03] kexec: Avoid overwriting the current pgd (V2, x86_64) | From | (Eric W. Biederman) | Date | Fri, 26 May 2006 10:32:19 -0600 |
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"Magnus Damm" <magnus.damm@gmail.com> writes:
> The code is not broken. The code does exactly what you are talking > about. Maybe I was a bit unclear.
Ack. I got a little confused. I was still thinking of what the identity mapping was trying to achieve, and you don't achieve the same thing.
> "page_table_a" contains two mappings. One which is the same as where > the page is mapped in the kernel (the virtual address where > relocate_new_kernel is located), and one identity mapped. So when the > switch occurs the cpu will continue to run on the same virtual > address.
In the general case which you want to cover page_table_a is still slightly wrong. In particular you can theoretically get a conflict between the virtual address of your page table switching function and the physical address of the control code page.
However while I agree that you need to do this in assembly for control I disagree that this code should be part of the relocate_new_kernel function.
Please move the code that uses page_table_a to a separate function, that when it is done jumps to the control_code page. Then you can map this page both virtually and physically with a statically allocated page table built a compile time.
This is a little simpler as you don't need to build this first page table dynamically and a little clearer as you aren't trying to get the control code page to serve two different functions.
If this function was more than 3 lines of assembly it could even be written in C for clarity with a special section so that the linker would not map it twice. Of course that would still need to address the stack usage problem.
>> So you need at least one additional entry in your page table. >> >> The fact that this bug did not jump out is a clear sign you were >> changing too many things at once, and did not have an adequate >> explanation in your change log. > > I will ignore that last part for now. I've tested the code on i386, > i386/pae, x86_64 (both opteron and athlon64) and all the combinations > with and without xen, and all configurations except x86_64 with both > kexec and kdump.
Testing is important here. But given that 99% of the bug hunting must be done via code review and thinking about the problem testing is not sufficient.
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