linux-kernel - Re: [PATCH v3 11/14] KVM: SVM: Introduce hybrid-AVIC mode

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Date:   Thu, 5 May 2022 10:57:36 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, joro@...tes.org,
        jon.grimm@....com, wei.huang2@....com, terry.bowman@....com
Subject: Re: [PATCH v3 11/14] KVM: SVM: Introduce hybrid-AVIC mode

Maxim

On 5/4/22 7:32 PM, Maxim Levitsky wrote:
> Well strictly speaking, another thing that has to be done, other that removing the inhibit,
> is to 'hide' the AVIC's private memslot if one of vCPUs is in x2apic mode,
> although not doing this doesn't cause any harm as the guest is not supposed to poke at xAPIC
> mmio even when uses x2apic, and if it does it will get the normal AVIC acceleration,
> so probably it is better to not add any more complexity and leave it like that.

Agree, I'll note this detail in the comment above.

> ... 
> BTW, hardware wise, does 'X2APIC_MODE' keeps the emulation of the AVIC mmio, or
> not?

According to the AVIC documentation, when the AVIC is configured to use x2 APIC mode
the MMIO access method will be disabled. This is consistent with the Intel x2APIC specification,
where it states "accessing the MMIO interface when in x2APIC mode, the behavior is identical
to xAPIC in globally disabled state".

Regards,
Suravee