Date: Mon, 19 May 2008 10:37:11 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Mike Meyer <mwm-keyword-freebsdhackers2.e313df@mired.org>, Rui Paulo <rpaulo@freebsd.org> Subject: Re: rdmsr from userspace Message-ID: <200805191037.12130.jhb@freebsd.org> In-Reply-To: <20080518152945.60989b9c@bhuda.mired.org> References: <482E93C0.4070802@icyb.net.ua> <48307700.70304@FreeBSD.org> <20080518152945.60989b9c@bhuda.mired.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sunday 18 May 2008 03:29:45 pm Mike Meyer wrote: > On Sun, 18 May 2008 19:35:44 +0100 > > Rui Paulo <rpaulo@FreeBSD.org> wrote: > > Mike Meyer wrote: > > > On Sun, 18 May 2008 16:50:28 +0100 > > > > > > Rui Paulo <rpaulo@FreeBSD.org> wrote: > > >> Mike Meyer wrote: > > >>> On Sat, 17 May 2008 11:13:52 +0300 > > >>> > > >>> Andriy Gapon <avg@icyb.net.ua> wrote: > > >>>> It seems that rdmsr instruction can be executed only at the highest > > >>>> privilege level and thus is not permitted from userland. Maybe we > > >>>> should provide something like Linux /dev/cpu/msr? > > >>>> I don't like interface of that device, I think that ioctl approach > > >>>> would be preferable in this case. > > >>>> Something like create /dev/cpuN and allow some ioctls on it: > > >>>> ioctl(cpu_fd, CPU_RDMSR, arg). > > >>>> What do you think? > > >>> > > >>> Ok, this points directly at a question I've been wondering about, but > > >>> haven't been able to find an answer in the google. > > >>> > > >>> I've been mucking about with general access to sysctl's (a sysctl > > >>> plugin for gkrellm, and a python module for accessing sysctls), and > > >>> with that hammer in my hand, the nail for this problem is obviously a > > >>> dev.cpu.#.msr sysctl. > > >> > > >> How can you request a rdmsr within the sysctl tree? I don't think > > >> sysctl is appropriate here either. > > > > > > Reading (or writing) a sysctl mib can trigger a sysctl handler, which > > > can do pretty much anything. In particular, there are already examples > > > in the kernel where sysctl handlers use devices that don't have /dev > > > entries to get & set their values. Look through kern/kern_cpu.c and > > > i386/cpufreq/p4tcc.c to see the two ends of that kind of > > > connection. In fact, the cpu frequency sysctls would seem to be an > > > excellent model for something like the msr. > > > > > > ioctl, open+read/write, sysctl - they're all just interfaces to kernel > > > handlers. > > > > > > <mike > > > > Yes, sure, but who do you select the MSR you want to read or write? > > > > dev.cpu.N.<insert MSR number in hexadecimal here> ? > > I don't think that would work - you'd have to register all those > hexadecimal strings as sysctl names. You could do an interface like > this, but the calling program would have to use sysctlnametomib to get > dev.cpu.N.msr, then append the MSR number to the results. Not really > very pretty. If you want to allow the user to poke at arbitrary msrs, > this would be a way to do it with sysctls, but the file api is > probably better. Actually, you don't have to register all of them. You can write a node handler which parses the next item in the MIB directly. Look at the all the proc sysctl's which accept a PID for example. That said, I think if someone already has a device driver done that is fine. I also think it is ok to let root request arbitrary MSR's from userland using an ioctl() or the like. -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200805191037.12130.jhb>