Re: [Xen-devel] [PATCH 1/3] AMD/IOMMU: don't needlessly trigger errors/crashes when unmapping a page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.11.2019 18:12, Andrew Cooper wrote:
> On 06/11/2019 15:18, Jan Beulich wrote:
>> Unmapping a page which has never been mapped should be a no-op (note how
>> it already is in case there was no root page table allocated).
> 
> Which function are you talking about here?  iommu_pde_from_dfn() will
> BUG() if no root was set up.

amd_iommu_unmap_page() has such a check first thing.

>> There's
>> in particular no need to grow the number of page table levels in use,
>> and there's also no need to allocate intermediate page tables except
>> when needing to split a large page.
> 
> To be honest, I've never been convinced that dynamically changing the
> number of levels in the AMD IOMMU tables is clever.  It should be fixed
> at 4 (like everything else) and suddenly a lot of runtime complexity
> disappears.  (I'm fairly confident that we'll need a domain create
> parameter to support 5 level paging in a rational way, so we won't even
> include walk-length gymnastics then either.)

5-level paging for the CPU 1st-stage-translation is imo pretty orthogonal
to needing 5 levels of paging for 2nd-stage-translation (which also is
what the IOMMU code here is about).

>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Thanks, Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel