Re: PROBLEM: 2.6 kernels on x86 do not preserve FPU flags across context switches

[eliot]

Hi Andi,

	you asked:
| On what CPUs does the failure occur? Linux uses different paths
| depending on if the CPU supports SSE or not.

Travis responded:

| We run on both AMDs (Durons and Athlons) as well as PII, PIII, and 
| PIV's. Our kernels are all compiled as generic 586+. Though when we were 
| testing for this, we did try the more generic 486+ option, as well as 
| exact processor matches for the AMD at least. I don't remember it making 
| a difference.

+-----------------------------
| Date:	Wed, 16 Jun 2004 23:40:18 +0200
| From:	Andi Kleen <ak@xxxxxx>
| Subject:	Re: PROBLEM: 2.6 kernels on x86 do not preserve FPU flags across context switches



| eliot@xxxxxxxxxx writes:

| > 	I am the team lead and chief VM developer for a Smaltalk
| > 	implementation based on a JIT execution engine.  Our customers
| > 	have been seeing rare incorrect floating-point results in
| > 	intensive fp applications on 2.6 kernels using various x86
| > 	compatible processors.  These problems do not occur on
| > 	previous kernel versons.  We recently had occasion to
| > 	reimplement our fp primitives to avoid severe performance
| > 	problems on Xeon processors that were traced to Xeon's
| > 	relatively slow implementation of fnclex and fstsw.  The older

| Funny, Linux just added fnclex to a critical path on popular request.
| But I guess it will need to be removed again, we already discussed
| that. 


| > I don't know whether any action on your part is appropriate.  The
| > use of the FPU status flags is presumably rare on linux (I believe
| > that neither gcc nor glibc make use of them).  But "exotic"
| > execution machinery such as runtimes for dynamic or functional
| > languages (language implementations that may not use IEEE arithmetic
| > and instead flag Infs and NaNs as an error) may fall foul of this
| > issue.  Since previous versions of the kernel on x86 apparently do
| > preserve the FPU status flags perhaps its simple to preserve the old
| > behaviour.  At the very least let me suggest you document the
| > limitation.

| This sounds like a serious kernel bug that should be fixed if
| true. Can you perhaps create a simple demo program that shows the
| problem and post it?

| On what CPUs does the failure occur? Linux uses different paths
| depending on if the CPU supports SSE or not.

| Does your program receive signals? Could it be related to them?

| -Andi
---
Eliot Miranda                 ,,,^..^,,,                mailto:eliot@xxxxxxxxxx
VisualWorks Engineering, Cincom  Smalltalk: scene not herd  Tel +1 408 216 4581
3350 Scott Blvd, Bldg 36 Suite B, Santa Clara, CA 95054 USA Fax +1 408 216 4500


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