Re: [Xen-devel] early_cpu_init() and identify_cpu()

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 17/7/07 11:00, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>> Does this mean that defaulting to setting up WC on a power-of-two-sized
>> region starting at the framebuffer address is not really safe on a fair
>> number of systems?
> 
> So I suppose, also based on the fact that Linux is defensive here too in
> defaulting to not touching the MTRRs at all. I implemented this in a similar
> fashion for Xen now.

How about remapping the framebuffer specifying WC in the PAT bits, if the
CPU is detected to support PAT? This might work more safely because we can
map at 4kB granularity rather than merely power-of-two. It depends on how
close the command queues actually are to the lfb in the VRAM map.

If we just mapped the 4kB-rounded region specified by lfb_base to
lfb_base+lfb_size (as determined via the VBE Get Mode Info call) as WC,
would that be safe?

If so we could use that unconditionally and avoid any MTRR-poking code. PAT
has been around for ages now.

 -- Keir


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