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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 17:50:54 +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 16:51:07 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 20/09/2019 14:54, 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>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
I'll rebase my CPUID series over this change.
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