Date: Wed, 22 Nov 2000 14:13:24 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: Terry Lambert <tlambert@primenet.com> Cc: arch@FreeBSD.org, (Jonathan Lemon) <jlemon@flugsvamp.com>, (Alfred Perlstein) <bright@wintelcom.net>, (Daniel Eischen) <eischen@vigrid.com> Subject: Re: Thread-specific data and KSEs Message-ID: <XFMail.001122141324.jhb@FreeBSD.org> In-Reply-To: <200011222206.PAA04455@usr07.primenet.com>
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On 22-Nov-00 Terry Lambert wrote: >> > > >> %fs and %gs, but no other seg regs. >> > > > >> > > > All I need is one. >> > > >> > > Well, %gs would cover x86 and k64. I think ia64 has several >> > > application registers that are available for OS use and we >> > > could steal one of those. I'm not sure about the alpha though. >> > >> > Don't more segment registers cause more overhead for context switches? >> >> It's just one more register that has to be saved. I don't >> think it's going to matter much. > > The %gs register already has to be saved for WINE processes, > so it's taken (at least when USER_LDT is defined). So there > would not be an additional context switch for it. Ok. Since %fs is only used in the kernel and is saved/restored it might be a good thing to use instead. > I think that if you guys go forward with this, you should do an > indirect through whatever you end up using. I realize this will > cost an additional 6 clock cycles, but it will let you expand > the list of things indefinitely, going forward, instead of having > to keep a register dedicated for backward compatability, and then > somehow "grow a new one" when you need to do something similar to > this again, in the future. It will be an indirect if I have any say in it. :) Currently we use %fs in the kernel to address a segment that contains per-CPU data. I think that if we use a seg reg, then we should have it address a segment that contains per-KSE data. > Terry Lambert > terry@lambert.org -- John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/ PGP Key: http://www.baldwin.cx/~john/pgpkey.asc "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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