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Re: [PATCH] domain: add barrier in vcpu_create()
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 26 Nov 2025 12:17:03 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 26 Nov 2025 11:17:30 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26.11.2025 12:05, Juergen Gross wrote:
> On 26.11.25 11:13, Jan Beulich wrote:
>> The lock-less list updating isn't safe against racing for_each_vcpu(),
>> unless done (by hardware) in exactly the order written.
>>
>> Fixes: 3037c5a2cb82 ("arm: domain")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> The Fixes: tag is pretty arbitrary; the issue was becoming non-latent when
>> Arm support was added. (Strictly speaking IA-64 and PPC would have been
>> affected too afaict, just that now that doesn't matter anymore [or, for
>> PPC, not yet, seeing that its support is being re-built from scratch].)
>>
>> I'm not quite happy about prev_id being plain int, but changing it to
>> unsigned (with suitable other adjustments) actually makes gcc15 generate
>> worse code on x86.
>>
>> --- a/xen/common/domain.c
>> +++ b/xen/common/domain.c
>> @@ -464,6 +464,7 @@ struct vcpu *vcpu_create(struct domain *
>> prev_id--;
>> BUG_ON(prev_id < 0);
>> v->next_in_list = d->vcpu[prev_id]->next_in_list;
>> + smp_wmb();
>> d->vcpu[prev_id]->next_in_list = v;
>> }
>>
>>
>
> It should be noted that this is an issue only in case for_each_vcpu() is
> running against the idle domain during cpu hotplug.
>
> All other domain get the vcpus populated form vcpu 0 upwards, so
> v->next_in_list will always be NULL for a new vcpu of a "normal" domain.
Hmm, right. And I don't think we ever do for_each_vcpu() on the idle domain.
Jan
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