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[Xen-devel] Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction abstraction

Linus Torvalds wrote:
On Wed, 18 Jun 2008, Jeremy Fitzhardinge wrote:
Along the lines of:

Hell no. There's a reason we have a special set_wrprotect() thing. We can do it more efficiently on native hardware by just clearing the bit atomically. No need to do the cmpxchg games.

It's not cmpxchg, just xchg.
In other words, is:

        lock btr $_PAGE_BIT_RW, (%rbx)

much cheaper than

        mov     $0, %rax
        xchg    %rax, (%rbx)
        and     $~_PAGE_RW, %rax
        mov     %rax, (%rbx)


It's the same number of locked RMW operations, so aside from being a few instructions longer, I think it would be much the same.

I guess the correct answer is "lmbench".


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