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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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".

   J

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