[coyotos-dev] Fixing the kernel heap size

[Sam Mason]

On Thu, Dec 20, 2007 at 06:25:49AM -0500, Godfrey Vassallo wrote:
> More recently, PCI bus devices usually provided a memory window whose
> size can be configured. Access to different portions of the device's
> memory sometimes required moving the window, which was accomplished
> through a dialog between the host based device driver and the device
> based firmware. In practice your 1G example may be easier to manage
> by placing a maximum window size (e.g. 2M) where possible on the
> device memory. This is somewhat in keeping with Landau's suggestion to
> restrict the use of device memory.

I'm confused.  I thought Shap's problem was with storing the kernel
related structures associated with device memory, not with the kernel
attempting to map the the actual device's memory itself.  My current
mental model of how it works is that the device memory would be mapped
into the virtual address space of the driver, but wouldn't be touched by
the kernel at all.

I can't imagine these growing without bound.  Having never had to look
after a system with hot-pluggable memory/PCI cards I'm not sure how much
this is ever actually done in practice.  I can't imagine devices getting
moved around much, if at all, after system install time.  I've also just
thought that the maximum number of CPUs supported by the kernel is fixed
at build time, it therefore doesn't seem insane to fix the number of
usable memory-mapped devices at boot time.

Then again, I may have interpreted things incorrectly! :)


  Sam