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Re: [Xen-devel] [PATCH] xen/sched: Introduce domain_vcpu() helper
- To: Xen-devel <xen-devel@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 23 Jan 2019 15:02:38 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Norbert Manthey <nmanthey@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 23 Jan 2019 15:03:19 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 23/01/2019 14:59, Andrew Cooper wrote:
> The progression of multi-vcpu support in Xen (originally a single pointer,
> then an embedded d->vcpu[] array, then a dynamically allocated array) has
> resulted in a large quantity of ad-hoc code for looking a vcpu up by id, and a
> large number of ways that the toolstack can cause Xen to trip over a NULL
> pointer. Some of this has been addressed in Xen 4.12, and work is ongoing.
>
> Another property of looking a vcpu up by id is frequently done in unprivileged
> hypercall context, making it an attractive target for speculative sidechannel
> attacks.
>
> Introduce a helper to do the lookup correctly, and without speculative
> interference. For performance reasons, it is useful not to have an smp_rmb()
> in this helper on ARM, and luckily this is safe to do, because of the
> serialisation offered by the global domheap lock.
>
> As a minor change noticed when checking the safety of this construct, sanity
> check during boot that idle->max_vcpus is a suitable upper bound for
> idle->vcpu[].
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: Norbert Manthey <nmanthey@xxxxxxxxx>
And in my haste, I forgot to tag this as "for 4.12".
~Andrew
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