Re: [Xen-devel] [PATCH 2/3] x86/apic: include the LDR when clearing out APIC registers

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.09.2019 16:01, Jan Beulich wrote:
> Although APIC initialization will typically clear out the LDR before
> setting it, the APIC cleanup code should reset the LDR.
> 
> This was discovered with a 32-bit KVM guest jumping into a kdump
> kernel. The stale bits in the LDR triggered a bug in the KVM APIC
> implementation which caused the destination mapping for VCPUs to be
> corrupted.
> 
> Note that this isn't intended to paper over the KVM APIC bug. The kernel
> has to clear the LDR when resetting the APIC registers except when X2APIC
> is enabled.
> 
> Signed-off-by: Bandan Das <bsd@xxxxxxxxxx>
> [Linux commit 558682b5291937a70748d36fd9ba757fb25b99ae]
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

FTR, commit 950b07c14e8c59444e2359f15fd70ed5112e11a0 reverts
this in Linux, but only for breaking offlining and then re-
onlining the BSP. Since we don't support this (yet), I don't
see a reason for us to revert as well.

Jan

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