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Re: [Xen-devel] an assertion triggered when running Xen on a HSW desktop
- To: <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 15 Jan 2019 11:59:01 +0000
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- Delivery-date: Tue, 15 Jan 2019 11:59:42 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 15/01/2019 11:50, Jan Beulich wrote:
>
>>> And even
>>> if I re-phrased your reply to mean an arbitrary alignment / order
>>> less than 9, then populating this with such a smaller order is still
>>> fine, and requesting the IOMMU mapping with that smaller order
>>> is still not going to trip the ASSERT() in question.
>> But the caller is then forced to iterate over the region and populate
>> it with order 0 calls to guest_physmap_add_page, which introduces a
>> lot of overhead.
> How is the overhead going to be any smaller if it's not the caller
> but iommu_map() to do the looping?
Because the calculation of how/when to change superpage level is far
simpler when you've got a mapped table in your hand.
You're also reducing the quantity of validation logic, and the absolute
number of spinlocks which need to taken/released for a single arbitrary
sized map. Furthermore, with the legacy mapping interface currently
being used, you also reduce the quantity of IO-TLB Flushing.
~Andrew
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