Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level paging

[hpa@zytor.com]

On May 26, 2017 6:00:57 AM PDT, "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
>On Thu, May 25, 2017 at 04:24:24PM -0700, Linus Torvalds wrote:
>> On Thu, May 25, 2017 at 1:33 PM, Kirill A. Shutemov
>> <kirill.shutemov@linux.intel.com> wrote:
>> > Here' my first attempt to bring boot-time between 4- and 5-level
>paging.
>> > It looks not too terrible to me. I've expected it to be worse.
>> 
>> If I read this right, you just made it a global on/off thing.
>> 
>> May I suggest possibly a different model entirely? Can you make it a
>> per-mm flag instead?
>> 
>> And then we
>> 
>>  (a) make all kthreads use the 4-level page tables
>> 
>>  (b) which means that all the init code uses the 4-level page tables
>> 
>>  (c) which means that all those checks for "start_secondary" etc can
>> just go away, because those all run with 4-level page tables.
>> 
>> Or is it just much too expensive to switch between 4-level and
>5-level
>> paging at run-time?
>
>Hm..
>
>I don't see how kernel threads can use 4-level paging. It doesn't work
>from virtual memory layout POV. Kernel claims half of full virtual
>address
>space for itself -- 256 PGD entries, not one as we would effectively
>have
>in case of switching to 4-level paging. For instance, addresses, where
>vmalloc and vmemmap are mapped, are not canonical with 4-level paging.
>
>And you cannot see whole direct mapping of physical memory. Back to
>highmem? (Please, no, please).
>
>We could possible reduce number of PGD required by kernel. Currently,
>layout for 5-level paging allows up-to 55-bit physical memory. It's
>redundant as SDM claim that we never will get more than 52. So we could
>reduce size of kernel part of layout by few bits, but not definitely to
>1.
>
>I don't see how it can possibly work.
>
>Besides difficulties of getting switching between paging modes correct,
>that Andy mentioned, it will also hurt performance. You cannot switch
>between paging modes directly. It would require disabling paging
>completely. It means we loose benefit from global page table entries on
>such switching. More page-walks.
>
>Even ignoring all of above, I don't see much benefit of having per-mm
>switching. It adds complexity without much benefit -- saving few lines
>of
>logic during early boot doesn't look as huge win to me.

It also makes no sense – the kernel threads only need one common page table anyway.
-- 
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