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


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 6 Nov 2019 17:12:42 +0000
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
  • Delivery-date: Wed, 06 Nov 2019 17:13:02 +0000
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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.

> 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.)

>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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