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Re: [PATCH v2 08/18] IOMMU/x86: support freeing of pagetables
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Thu, 2 Dec 2021 17:03:56 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx>, Paul Durrant <paul@xxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Thu, 02 Dec 2021 16:04:24 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Fri, Sep 24, 2021 at 11:48:21AM +0200, Jan Beulich wrote:
> For vendor specific code to support superpages we need to be able to
> deal with a superpage mapping replacing an intermediate page table (or
> hierarchy thereof). Consequently an iommu_alloc_pgtable() counterpart is
> needed to free individual page tables while a domain is still alive.
> Since the freeing needs to be deferred until after a suitable IOTLB
> flush was performed, released page tables get queued for processing by a
> tasklet.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> I was considering whether to use a softirq-taklet instead. This would
> have the benefit of avoiding extra scheduling operations, but come with
> the risk of the freeing happening prematurely because of a
> process_pending_softirqs() somewhere.
Another approach that comes to mind (maybe you already thought of it
and discarded) would be to perform the freeing after the flush in
iommu_iotlb_flush{_all} while keeping the per pPCU lists.
That would IMO seem better from a safety PoV, as we know that the
flush has been performed when the pages are freed, and would avoid the
switch to the idle domain in order to do the freeing.
Thanks, Roger.
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