RE: [PATCH v3 20/23] VT-d: free all-empty page tables

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Tuesday, January 11, 2022 12:36 AM
> 
> When a page table ends up with no present entries left, it can be
> replaced by a non-present entry at the next higher level. The page table
> itself can then be scheduled for freeing.
> 
> Note that while its output isn't used there yet,
> pt_update_contig_markers() right away needs to be called in all places
> where entries get updated, not just the one where entries get cleared.
> 
> Note further that while pt_update_contig_markers() updates perhaps
> several PTEs within the table, since these are changes to "avail" bits
> only I do not think that cache flushing would be needed afterwards. Such
> cache flushing (of entire pages, unless adding yet more logic to me more
> selective) would be quite noticable performance-wise (very prominent
> during Dom0 boot).
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v3: Properly bound loop. Re-base over changes earlier in the series.
> v2: New.
> ---
> The hang during boot on my Latitude E6410 (see the respective code
> comment) was pretty close after iommu_enable_translation(). No errors,
> no watchdog would kick in, just sometimes the first few pixel lines of
> the next log message's (XEN) prefix would have made it out to the screen
> (and there's no serial there). It's been a lot of experimenting until I
> figured the workaround (which I consider ugly, but halfway acceptable).
> I've been trying hard to make sure the workaround wouldn't be masking a
> real issue, yet I'm still wary of it possibly doing so ... My best guess
> at this point is that on these old IOMMUs the ignored bits 52...61
> aren't really ignored for present entries, but also aren't "reserved"
> enough to trigger faults. This guess is from having tried to set other

Is this machine able to capture any VT-d faults before? If yes maybe
you will observe more information if trying to tweak those bits at a later
time (instead of when iommu is enabled)?

Thanks
Kevin