Messages in this thread | | | Date | Mon, 30 Oct 2006 14:07:50 -0500 | From | Phillip Susi <> | Subject | Re: Avoidable floating point save/restore? |
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The FP registers only need to be saved/restored if one or both of the current task and the task being switched to have made use of them ( i.e. they ever do FP or MMX math ). Initially FP access is disabled and the first time a task tries to use the FPU, a fault is triggered and the kernel enables the FPU for that task and sets a flag so it remembers it needs to save/restore the state when switching in/out of that task.
John Richard Moser wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > I found this from comp.os.minix (actually part of a MINIX FAQ): > > =====CUT===== > From: kjb=733301@cs.vu.nl (Kees J Bot) > Subject: Re: MMX/3DNow support was RE: MINIX Development? > Date: Wed, 23 Jul 2003 20:15:03 +0200 > > This is really a hardware floating point issue, because the MMX > registers share the FP registers. This was done so that MMX unaware OSen > can still support MMX programs, because when they save and restore the > FP registers then the MMX state is also saved and restored if that > happens to be what the FP registers are used for. This saving and > restoring is what Minix doesn't do. So if two processes use FP/MMX then > a context switch from one to the other will clobber the FP state of > both. What is needed to make this work is a trap handler that reacts to > the use of FP, so that Minix can save the FP state of the process that > last used FP and load the FP state of the current process. On a context > switch Minix merely sets the "don't use FP" bit in some register. Costs? > One FP interrupt handler, some FP save/restore/setup code, some memory > per process to store the FP state into, and some memory to store the FP > state when a user process catches a signal. (Not sure about the signal > business, much check with Philip.) This isn't much work, we can simply > take Minix-vmd's code, but I haven't seen any need yet. Minix has to use > software FP as distributed, or it won't run on your old 386, so Minix > itself doesn't need it. Anyone here who wants to use Minix for some > heavy number crunching? If so then I could be persuaded to add an > ENABLE_FPU to the next release, by default off. I don't care about MMX, > that's way too exotic for Minix. > =====CUT===== > > I'm trying to make heads or tails about what in the heck is going on > here. It looks like they're saying you don't need to save/restore FP > registers between context switches unless one process uses FP and the > other uses MMX; but that doesn't make ANY sense at all. If > gnome-session divides 3.14/2.28 and then gimp divides 3.33/2.22 and then > we switch BACK to gnome-session and it wants to divide the result by > 1.92, wouldn't we need the FPU registers back in the exact state they > were at before switching away? > > - -- > We will enslave their women, eat their children and rape their > cattle! > -- Bosc, Evil alien overlord from the fifth dimension > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.3 (GNU/Linux) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org > > iQIVAwUBRUYgNAs1xW0HCTEFAQJnqBAAgMjqTSA0OC6ThwUSoG4lO0uFCdUERJKl > O0Mxtq9XqKOU4W7XnCtIkBF0Gy1HdMm8/L1MXLRM0FMv6jz17nNb3TGvLRC/4KKh > vgl4KN/2D6tYYdKyh+v0n/zgVYGJ0lZn3sNQdBhxXet1Zjm6n7REFvJJD07PzIcm > XK5vciaVHYfNb0N3SdSL7JBJdMHzrbq2DmqUD5OsVOok37e1F3S4+T3EBZy+mk/i > D6lcx2KJzp8vIWmoskN+WLBox8fihSoNtxOK/oLComHwyJ3z1FEuILTvYqEjGUGg > k1TdEQYhTrFCjkpLgK5lhn1hD8dZRldN4bVEt9uWU5WiRCGiykPog3wlVz7T6bQr > SOAVKA4157kfDvG3KY55pNTxy0j8TgBWQhPyh/dTcRlzYbZuqccRcHQbYyBEGByZ > Of7tVUnHzgNd8kuzWqVh4ULDJ7YyNj+INFegoluLi/3JcQFL/EhoUYX/QNbX1VK1 > vkrwGOneAC2KttjaH/cZHUuYfwZ1hkMmDjbkHlGmb4z56TRKOtUt03ziMDDhO4Fv > WWjVn+CdAiOTkhuVgIQ+EpnNCSP/yRUTmxDrI3+KJaIW6zlaAZyS29KoWA+EH75K > mJldV7amMcD+6KGVP7tHuTOpGXKYa2tCCfwtGI65Z7rWvGD6RUzz+waiT4kPav1G > 1lObgtnarqY= > =fw5+ > -----END PGP SIGNATURE----- > -
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