Re: Add 'info pg' command to monitor

[Peter Maydell]

On Mon, 15 Apr 2024 at 17:09, Don Porter <porter@cs.unc.edu> wrote:
> I am a CS professor (and, newly, a second-time contributor). I have
> been using qemu in my courses for over a decade, especially a course
> that asks students to write major pieces of an OS kernel from starter
> code.
>
> I have some patches, originally from Austin Clements at MIT, that I
> have found useful over the years and that may be useful to others.  It
> would also be nice not to have to build a custom qemu each semester.  I
> have cleared upstreaming these with Austin, the original author.
>
> This patch set adds an 'info pg' command to the monitor, which prints
> a nicer view of the page tables.  A project in my graduate OS course
> involves implementing x86 page table support, and my students have
> found this helpful for debugging.

So, my issue with this is that it's x86 specific, and it adds
yet another monitor command that is doing "show some kind of debug
info related to the guest page tables", along with "info mem"
and "info tlb". Plus it is yet another lump of code that's
walking the guest page tables and interpreting them.

What I'd really like to see is some infrastructure that is
at least somewhat guest-architecture-agnostic, so we can
minimise what a guest architecture needs to implement (and
then make providing that mandatory).

The other thing I'd like to see is perhaps some investigation of
whether there's any way to implement something useful by
using/extending the existing get_phys_page_attrs_debug() and
similar functions, so that you don't have to write one lot
of page-table-walking code for QEMU to use when it's executing
guest code and a separate lot (that's bound to get out of
sync or not support new functionality/changes) that's only
handling these monitor debug commands. There's a lot of
complexity in figuring out things like permissions in a
modern architecture...

thanks
-- PMM