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Re: [PATCH 1/9] gnttab: defer allocation of maptrack frames table
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 6 Sep 2021 15:05:28 +0100
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- Cc: George Dunlap <george.dunlap@xxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26/08/2021 11:09, Jan Beulich wrote:
> By default all guests are permitted to have up to 1024 maptrack frames,
> which on 64-bit means an 8k frame table. Yet except for driver domains
> guests normally don't make use of grant mappings. Defer allocating the
> table until a map track handle is first requested.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Nack. This creates new dynamic failures outside the VM's control,
therefore regressing Xen's usability.
The maptrack array (and frames for that matter) should be allocated at
domain creation time, like we do for most other structures in the
hypervisor.
Furthermore, seeing as this has come up on multiple community calls, I
will remind you that it is not just Citrix as a downstream which is
firmly of this opinion.
This entire patch should be replaced with one that...
> ---
> I continue to be unconvinced that it is a good idea to allow all DomU-s
> 1024 maptrack frames by default.
... defaults to something smaller for plain domUs, because this improves
the general case without leaving VMs more liable to crash at runtime.
~Andrew
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