On 12/12/2015 04:05 PM, Segher Boessenkool wrote: > On Sat, Dec 12, 2015 at 03:01:26PM -0800, Haren Myneni wrote: >> On 12/12/2015 12:43 AM, Segher Boessenkool wrote: >>> On Fri, Dec 11, 2015 at 07:30:29PM -0800, Haren Myneni wrote: >>>> NX842 coprocessor sets 3rd bit in CR register with XER[S0] which is >>>> nothing to do with NX request. On powerpc, XER[S0] will be set if >>>> overflow in FPU and stays until another floating point operation is >>>> executed. Since this bit can be set with other valuable return status, >>>> ignore this XER[S0] value. >>> >>> XER[SO] is the *integer* summary overflow bit. It is set by OE=1 >>> instructions ("addo" and the like), and can only be cleared explicitly >>> (using "mtxer"). >> >> Thanks for the correct description. I was told XER[S0] is floating overflow from FPU. > > You can use the Power ISA document to make sure for yourself. > >>>> + if (ret & ICSWX_XERS0) >>>> + ret &= ~ICSWX_XERS0; >>> >>> You can just always clear it, there is no need to check if it is set first. >> >> Do you mean reset this before calling NX? > > I mean write this as simply > > + ret &= ~ICSWX_XERS0; > > (without any "if"). > >> I believe NX coprocessor should not set CR bit as XER[S0] nothing to do with NX request and it is no use. > > Many instructions set the CR bit to XER[SO] -- store conditional, slbfee., > and all "normal" dot insns and of course cmp[l][i]. Or, shorter: "everything" > does this. > >> NX is copying this CR bit with XER. But reset XER[S0] has to be done before NX request. > > Only if you care what the final value of bit 3 in the CR will be. Even > in the unusual case where you want to look at all CR field bits at once > it is cheap to just mask out the bit (as you do). > >> We can not do this in icswx since this instruction can be used by other coprocessors in future. But I am not comfortable clearing as we are not touching this XER in the driver or result of NX operation. So I am proposing this patch to fix this not proper NX behaviour - ignores CR bit. > > I really wouldn't call it "not proper", that makes it sound like there > is an implementation bug or design mistake. Instead, you could say that > your switch statement looks at the values of bits 0, 1, and 2, so you > just mask those -- you do not care about the value of bit 3, so you mask > it out. > > Something like > > + /* Mask out the bits we do not care about. */ > + ret &= ~ICSWX_XERS0; icswx RFC says CR[3] bit can be undefined or set to XER[S0]. So was thinking NX should have ignored since no use to user. I may be wrong. Thanks for your help. I will repost this patch with your suggestions. Thanks Haren > > > Segher > -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html