linux-kernel - Re: [PATCH v6 6/9] KVM: x86: lapic: don't allow to change APIC ID unconditionally

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Date:   Fri, 25 Feb 2022 14:56:02 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Zeng Guang <guang.zeng@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Robert Hu <robert.hu@...el.com>, Gao Chao <chao.gao@...el.com>
Subject: Re: [PATCH v6 6/9] KVM: x86: lapic: don't allow to change APIC ID
 unconditionally

On Fri, 2022-02-25 at 16:46 +0200, Maxim Levitsky wrote:
> On Fri, 2022-02-25 at 16:22 +0800, 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.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > Signed-off-by: Zeng Guang <guang.zeng@...el.com>
> 
> Assuming that this is approved and accepted upstream,
> that is even better that my proposal of doing this
> when APICv is enabled.
> 
> Since now apic id is always read only, now we should not 
> forget to clean up some parts of kvm like kvm_recalculate_apic_map,
> which are not needed anymore

Can we also now optimise kvm_get_vcpu_by_id() so it doesn't have to do
a linear search over all the vCPUs when there isn't a 1:1
correspondence with the vCPU index?

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