SYSENTER/SYSEXIT_vs._SYSCALL/SYSRET
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From: Rob Warnock on 22 Jan 2010 07:10 Paul A. Clayton <paaronclayton(a)embarqmail.com> wrote: +--------------- | ga...(a)allegro.com (Gavin Scott) wrote: | > PA-RISC has an even more flexible form of this in that you can | > associate a privilege promotion level with an executable page. | > The promotion does not happen when you branch to the page, but | > when you execute a PC relative branch instruction on that page | > that includes the ,GATE option, the target will execute at the | > set privilege level. So you can fill a page up with entry points | > that promote themselves. Promotion can only happen at the points | > of the GATE instructions, so branching into the middle of an | > instruction sequence won't let you do anything special. | > | > A page marked in this way is called a Gateway page, and this is | > the primary mechanism for privilege promotion in the architecture. | | Itanium (sort of descended from PA-RISC) has a Enter Privileged Code | instruction. Jumping to it elevates privilege (if it is in a page | that allows for such escalation). ... +--------------- The venerable DEC PDP-10 (well, not the KA10 CPU, but the KI10 and later) used PORTAL instructions[1] to protect entry points into read-protected shared hisegs. -Rob [1] IIRC, it was a JFCL with an otherwise-unused combination of flags. ----- Rob Warnock <rpw3(a)rpw3.org> 627 26th Avenue <URL:http://rpw3.org/> San Mateo, CA 94403 (650)572-2607
From: Ken Hagan on 22 Jan 2010 07:52 On Thu, 21 Jan 2010 20:05:07 -0000, Robert Myers <rbmyersusa(a)gmail.com> wrote: > On Jan 21, 12:51 pm, MitchAlsup <MitchAl...(a)aol.com> wrote: > >> This reminds me of what the physicists were probably talking about >> just after the turn of the previous century between the discovery of >> the photoelectric effect and the development of quantum mechanics. > > Not likely. One of the most easily-understood results of special > relativity is that a universal clock is not possible even in theory. Nitpick: The photo-electric effect was known before special relativity. In fact, according to wikipedia, special relativity was Einstein's *3rd* paper of the year, so even his *explanation* of the photo-electric effect preceeded his assertion that universal clocks aren't possible. However, I suspect that both you and Mitch are in violent agreement about the pursuit of wonderfully scalable clocks. A globally consistent high-performance timer isn't possible, so it really makes no sense either to try to provide it or to write software that needs it. If software needs several (logical) threads to agree on a defined order of events then it must arrange for those threads to meet "at or near" a particular location and agree to use the timer they find there.
From: nmm1 on 22 Jan 2010 08:33 In article <op.u6xp06gjss38k4(a)khagan.ttx>, Ken Hagan <K.Hagan(a)thermoteknix.com> wrote: > >However, I suspect that both you and Mitch are in violent agreement about >the pursuit of wonderfully scalable clocks. A globally consistent >high-performance timer isn't possible, so it really makes no sense either >to try to provide it or to write software that needs it. If software needs >several (logical) threads to agree on a defined order of events then it >must arrange for those threads to meet "at or near" a particular location >and agree to use the timer they find there. You can do better than that. But, as with almost all real problems, you have to sacrifice one characteristic if you want to push another to the limits. Regards, Nick Maclaren.
From: Bernd Paysan on 22 Jan 2010 10:07 Ken Hagan wrote: > On Thu, 21 Jan 2010 20:05:07 -0000, Robert Myers <rbmyersusa(a)gmail.com> > wrote: > >> On Jan 21, 12:51 pm, MitchAlsup <MitchAl...(a)aol.com> wrote: >> >>> This reminds me of what the physicists were probably talking about >>> just after the turn of the previous century between the discovery of >>> the photoelectric effect and the development of quantum mechanics. >> >> Not likely. One of the most easily-understood results of special >> relativity is that a universal clock is not possible even in theory. > > Nitpick: The photo-electric effect was known before special relativity. In > fact, according to wikipedia, special relativity was Einstein's *3rd* > paper of the year, so even his *explanation* of the photo-electric effect > preceeded his assertion that universal clocks aren't possible. As long as the clocks don't move, you don't have problems ;-). -- Bernd Paysan "If you want it done right, you have to do it yourself" http://www.jwdt.com/~paysan/
From: nmm1 on 22 Jan 2010 10:15
In article <2443919.qULjOqU9HR(a)elfi.zetex.de>, Bernd Paysan <bernd.paysan(a)gmx.de> wrote: >Ken Hagan wrote: >> On Thu, 21 Jan 2010 20:05:07 -0000, Robert Myers <rbmyersusa(a)gmail.com> >> wrote: >> >>> Not likely. One of the most easily-understood results of special >>> relativity is that a universal clock is not possible even in theory. >> >> Nitpick: The photo-electric effect was known before special relativity. In >> fact, according to wikipedia, special relativity was Einstein's *3rd* >> paper of the year, so even his *explanation* of the photo-electric effect >> preceeded his assertion that universal clocks aren't possible. > >As long as the clocks don't move, you don't have problems ;-). Actually, that's not true. The only prediction that general relativity has made that has been confirmed by direct evidence is the effect that time varies with gravitational potential. Regards, Nick Maclaren. |