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Re: [PATCH v2 08/18] IOMMU/x86: support freeing of pagetables
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 2 Dec 2021 17:10:38 +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 16:11:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.12.2021 17:03, Roger Pau Monné wrote:
> 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.
This was my initial plan, but I couldn't convince myself that the first
flush to happen would be _the_ one associated with the to-be-freed
page tables. ISTR (vaguely though) actually having found an example to
the contrary.
Jan
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