Messages in this thread | | | Date | Fri, 09 May 2003 21:00:46 +0000 | From | Philippe Elie <> | Subject | Re: [PATCH] Mask mxcsr according to cpu features. |
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paubert wrote: > On Fri, May 09, 2003 at 04:33:37PM +0000, Philippe Elie wrote:
>>The only problem we can get is an old processor which write non >>zero but random bits in the 16 upper bits. > > > I don't believe that there is any, but that maybe some which don't > write anything, hence the requirement for clearing the area in the > DAZ detection algorithm.
right
>>my documentation says to fxsave and get the features mask from >>the mxcsr mask but to fall back to 0xffbf if mask == 0, quoting >>docs 11.6.6: >> >>1 setup a fxsave area >>2 clear this area >>3 fxsave in this area >>4 if mxcsr == 0 use mask 0xffbf else use mxcsr mask > > > Too expensive unless the mask is computed at boot time once and for
yeps,
> all (thrashing half a kB for a single 32 bit constant, sigh). I did
uh? you just need to fxsave on stack, extract the mask, the struct is 512 bytes length, surely during kernel init 512 bytes stack allocation is right
> not want to touch too many files in my patch, but it seems unavoidable.
> Now a last question, are there SMP systems in which one processor > supports DAZ and the other does not, just to complicate matters a > little more?
Such system are not symetric. I don't think we must take care about this theorical things and I'm pretty sure than mixing old P4 and newers in a box can't work. Anyway even if it works userspace program using DAZ will be not reliable since they can run from time to time on cpu with DAZ then cpu w/o DAZ.
regards, Phil
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