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Re: [Xen-devel] [PATCH 3/3] x86/apic: do not initialize LDR and DFR for bigsmp
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 6 Sep 2019 15:03:02 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 06 Sep 2019 14:03:21 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 06/09/2019 15:01, Jan Beulich wrote:
> Legacy apic init uses bigsmp for smp systems with 8 and more CPUs. The
> bigsmp APIC implementation uses physical destination mode, but it
> nevertheless initializes LDR and DFR. The LDR even ends up incorrectly with
> multiple bit being set.
>
> This does not cause a functional problem because LDR and DFR are ignored
> when physical destination mode is active, but it triggered a problem on a
> 32-bit KVM guest which jumps into a kdump kernel.
>
> The multiple bits set unearthed a bug in the KVM APIC implementation. The
> code which creates the logical destination map for VCPUs ignores the
> disabled state of the APIC and ends up overwriting an existing valid entry
> and as a result, APIC calibration hangs in the guest during kdump
> initialization.
>
> Remove the bogus LDR/DFR initialization.
>
> This is not intended to work around the KVM APIC bug. The LDR/DFR
> ininitalization is wrong on its own.
>
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Bandan Das <bsd@xxxxxxxxxx>
> [Linux commit bae3a8d3308ee69a7dbdf145911b18dfda8ade0d]
>
> Drop init_apic_ldr_x2apic_phys() at the same time.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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