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rfc: a driver to access MSRs on i386

Subject: rfc: a driver to access MSRs on i386
From: Marc Balmer
Date: Sun, 24 Jan 2010 17:32:59 UTC
Newsgroups: fa.netbsd.tech.kern


While porting the xf86-video-geode X.Org driver to NetBSD I also needed to implement the amdmsr(4) driver I wrote for OpenBSD. The issue here is, that xf86-video-geode needs access to several MSRs of the AMD Geode LX CPU to program the graphics processor. amdmsr(4) is a driver that allows you todo just this. It is bound to this specific type of CPU, whence the strange name.

One problem of the driver is, that once it has matched, it allows access to all MSRs, which can be a security risk (but then, it only attaches on a AMD Geode LX CPU that has the graphics processor).

I want to extend this on NetNSD, making this a more general and thus more useable driver. This is what I have in mind:

- Rename the driver to msr(4)
- Attach always
- Disable access to MSRs by default
- Integrate with kauth(9) much in the way gpio(4) does:
* Access to MSRs must be defined (MSR address or range, read or write access) at a "low securelevel" (of course, we don't have a securelevel, but the kauht(9) equivalent thereof). Once the "securelevel" has been raised, only the defined MSRs can be accessed. * It is also possible to enable access to all MSRs (wildcard), useful for e.g. eval boards or general tinkering with the system.
- Access to MSRs is via a set of ioctls()
- A command msrctl(8) in /sbin allows the configuration of msr(4) at the "low securelevel" and also to access the (configured) MSRs

Comments and ideas welcome.

Marc Balmer


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