dean gaudet wrote:
On Sun, 9 Sep 2007, Nick Piggin wrote:
I've also heard that string operations do not follow the normal ordering, but
that's just with respect to individual loads/stores in the one operation, I
hope? And they will still follow ordering rules WRT surrounding loads and
stores?
see section 7.2.3 of intel volume 3A...
"Code dependent upon sequential store ordering should not use the string
operations for the entire data structure to be stored. Data and semaphores
should be separated. Order dependent code should use a discrete semaphore
uniquely stored to after any string operations to allow correctly ordered
data to be seen by all processors."
i think we need sfence after things like copy_page, clear_page, and
possibly copy_user... at least on intel processors with fast strings
option enabled.
I do not think. I believe that authors are trying to say that
struct { uint8 lock; uint8 data; } x;
lea (x.data),%edi
mov $2,%ecx
std
rep movsb
to set both data and lock does not guarantee that x.lock will be set
after x.data and that you should do
lea (x.data),%edi
std
movsb
movsb # or mov (%esi),%al; mov %al,(%edi), but movsb looks discrete
enough to me
instead (and yes, I know that my example is silly).
Petr
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