Re: [PATCH 6.6.y 2/2] KVM: x86: Re-split x2APIC ICR into ICR+ICR2 for AMD (x2AVIC)

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 05, 2025, James Houghton wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> Re-introduce the "split" x2APIC ICR storage that KVM used prior to Intel's
> IPI virtualization support, but only for AMD.  While not stated anywhere
> in the APM, despite stating the ICR is a single 64-bit register, AMD CPUs
> store the 64-bit ICR as two separate 32-bit values in ICR and ICR2.  When
> IPI virtualization (IPIv on Intel, all AVIC flavors on AMD) is enabled,
> KVM needs to match CPU behavior as some ICR ICR writes will be handled by
> the CPU, not by KVM.
> 
> Add a kvm_x86_ops knob to control the underlying format used by the CPU to
> store the x2APIC ICR, and tune it to AMD vs. Intel regardless of whether
> or not x2AVIC is enabled.  If KVM is handling all ICR writes, the storage
> format for x2APIC mode doesn't matter, and having the behavior follow AMD
> versus Intel will provide better test coverage and ease debugging.
> 
> Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
> Cc: stable@vger.kernel.org
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> Link: https://lore.kernel.org/r/20240719235107.3023592-4-seanjc@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> (cherry picked from commit 73b42dc69be8564d4951a14d00f827929fe5ef79)

Same nit on the upstream info here.  Don't think it warrants a v2? (that's a
question for Sasha and/or Greg).

Acked-by: Sean Christopherson <seanjc@google.com>