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Message-ID: <eb849245c98ea7f5d5e9320ee6ee6b0d1851b439.camel@infradead.org>
Subject: Re: [PATCH v6 6/9] KVM: x86: lapic: don't allow to change APIC ID
 unconditionally
From:   David Woodhouse <dwmw2@infradead.org>
To:     Maxim Levitsky <mlevitsk@redhat.com>,
        Zeng Guang <guang.zeng@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Kim Phillips <kim.phillips@amd.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jethro Beekman <jethro@fortanix.com>,
        Kai Huang <kai.huang@intel.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Robert Hu <robert.hu@intel.com>, Gao Chao <chao.gao@intel.com>
Date:   Fri, 25 Feb 2022 14:56:02 +0000
In-Reply-To: <79f5ce60c65280f4fb7cba0ceedaca0ff5595c48.camel@redhat.com>
References: <20220225082223.18288-1-guang.zeng@intel.com>
         <20220225082223.18288-7-guang.zeng@intel.com>
         <79f5ce60c65280f4fb7cba0ceedaca0ff5595c48.camel@redhat.com>
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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@redhat.com
> > >
> >=20
> > No normal guest has any reason to change physical APIC IDs, and
> > allowing this introduces bugs into APIC acceleration code.
> >=20
> > 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/
> >=20
> >=20
> > 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.
> >=20
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > Signed-off-by: Zeng Guang <guang.zeng@intel.com>
>=20
> Assuming that this is approved and accepted upstream,
> that is even better that my proposal of doing this
> when APICv is enabled.
>=20
> Since now apic id is always read only, now we should not=20
> 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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