linux-kernel - Re: [PATCH v8 6/9] KVM: x86: lapic: don't allow to change APIC ID unconditionally

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Date:   Fri, 15 Apr 2022 14:39:48 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zeng Guang <guang.zeng@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Kai Huang <kai.huang@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Robert Hu <robert.hu@...el.com>,
        Gao Chao <chao.gao@...el.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH v8 6/9] KVM: x86: lapic: don't allow to change APIC ID
 unconditionally

On Mon, Apr 11, 2022, Zeng Guang wrote:
> From: Maxim Levitsky <mlevitsk@...hat.com>
> 
> No normal guest has any reason to change physical APIC IDs, and
> allowing this introduces bugs into APIC acceleration code.
> 
> And Intel recent hardware just ignores writes to APIC_ID in
> xAPIC mode. More background can be found at:
> https://lore.kernel.org/lkml/Yfw5ddGNOnDqxMLs@google.com/
> 
> Looks there is no much value to support writable xAPIC ID in
> guest except supporting some old and crazy use cases which
> probably would fail on real hardware. So, make xAPIC ID
> read-only for KVM guests.

AFAIK, the plan is to add a capability to let userspace opt-in to a fully read-only
APIC ID[*], but I haven't seen patches...

Maxim?

[*] https://lore.kernel.org/all/c903e82ed2a1e98f66910c35b5aabdcf56e08e72.camel@redhat.com