Re: rfc: a driver to access MSRs on i386

[David Young <dyoung%pobox.com@localhost>]

On Sun, Jan 24, 2010 at 06:32:32PM +0100, Marc Balmer wrote:
> 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.

Does the driver really need to access the MSRs of the Geode LX Graphics
Processor?  Maybe there is some mistake.  Text in the data book suggests
that touching the MSRs is probably unnecessary:

        The Standard GLD MSRs (accessed via the RDMSR and WRMSR
        instructions) control the Graphics Processor's behavior as
        a GLIU module. These registers should be programmed at
        configuration time and left alone thereafter. They do not
        need to be modified by software to set up any of the graphics
        primitives.

(AMD Geode(tm) LX Processors Data Book, 6.4 Graphics Processor
Register Definitions, p. 254)

It sounds to me that the BIOS or the kernel should initialize the MSRs
and then leave them be.  One of the GP MSRs, BTW, controls the base
address of a 16M "command buffer," a contiguous, non-cacheable physical
memory region (p. 239).

> 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:

Why not provide MI drivers for the abstract functions (e.g., graphics
display, audio playback, encryption/decryption) controlled by these MSRs
instead of for the MSRs themselves?

Can you do without an ioctl for invoking rdmsr/wrmsr if you trap the
instructions and simulate them in the kernel?

Dave

-- 
David Young             OJC Technologies
dyoung%ojctech.com@localhost      Urbana, IL * (217) 278-3933