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Re: [Xen-devel] [PATCH v2] libxc/x86: avoid overflow in CPUID APIC ID adjustments
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 23 Sep 2019 18:33:27 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 23 Sep 2019 17:33:41 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 23/09/2019 09:29, Jan Beulich wrote:
> On 20.09.2019 15:54, Jan Beulich wrote:
>> Recent AMD processors may report up to 128 logical processors in CPUID
>> leaf 1. Doubling this value produces 0 (which OSes sincerely dislike),
>> as the respective field is only 8 bits wide. Suppress doubling the value
>> (and its leaf 0x80000008 counterpart) in such a case.
>>
>> Note that while there's a similar overflow in intel_xc_cpuid_policy(),
>> that one is being left alone for now.
>>
>> Note further that while it was considered to suppress the multiplication
>> by 2 altogether if the host topology already provides at least one bit
>> of thread ID within APIC IDs, it was decided to avoid more change here
>> than really needed at this point.
>>
>> Also zap leaf 4 (and at the same time leaf 2) EDX output for AMD, as it
>> should have been from the beginning.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> v2: Drop use of physinfo. Drop Intel-only leaf 4 change. Increment
>> ApicIdCoreSize only when doubling NumberOfCores.
> Thinking about it some more, dropping the leaf 4 change seems at least
> somewhat risky to me. This being just a 6-bit field (and effectively
> already saturating in a way, at least when power-of-two maximum core
> counts are involved), and hence there being 2 bits of "playing room"
> between this and the involved leaf 1 field, the calculation there not
> getting adjusted is still a latent risk imo with guest side calculations
> like this one
>
> smp_num_siblings = smp_num_siblings / c->x86_max_cores;
>
> found in basically all versions of Linux (where certain functions,
> e.g. apic_id_is_primary_thread(), won't cope with smp_num_siblings
> ending up as zero, while others, e.g. topology_smt_supported(), do).
None of these adjustments are correct. The topology should be
constructed from first principles.
What matters is having as few alterations to the algorithm as necessary,
until we can fix the many toolstack side issues.
~Andrew
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