Messages in this thread | | | Date | 17 Jun 2004 12:35:29 +0200 | Date | Thu, 17 Jun 2004 12:35:29 +0200 | From | Andi Kleen <> | Subject | Re: PROBLEM: 2.6 kernels on x86 do not preserve FPU flags across context switches |
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On Wed, Jun 16, 2004 at 04:01:46PM -0700, eliot@cincom.com wrote: > 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
And you saw it on all of them? (in particular both on PII and on PIV?)
I actually doubt the problem happens on a context switch - the kernel just uses FXSAVE/FNSAVE for this and this is extremly hard to get wrong. Either you have no FPU state saved at all or you have all, since the CPU handles it completely in microcode.
Most likely candidate would be signal context saving. When a signal happens and the process used floating point then the i386 kernel converts the internal FXSAVE/FNSAVE image to another image derived from iBCS on the signal stack (and then later back). If any problems with subtle corruptions happen I would expect them in this process.
This would be more likely on SSE enabled CPUs though, on pre SSE CPUs this code is much simpler.
Do you know which status bit gets corrupted exactly?
-Andi
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