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Subject: Re: [PATCH v5 7/8] KVM: VMX: Update PID-pointer table entry when
 APIC ID is changed
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Chao Gao <chao.gao@intel.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     Zeng Guang <guang.zeng@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Paolo Bonzini <pbonzini@redhat.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" <kvm@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Luck, Tony" <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>,
        "Huang, Kai" <kai.huang@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Hu, Robert" <robert.hu@intel.com>
Date:   Wed, 23 Feb 2022 12:26:31 +0200
In-Reply-To: <20220223061037.GA21263@gao-cwp>
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On Wed, 2022-02-23 at 14:10 +0800, Chao Gao wrote:
> On Thu, Feb 03, 2022 at 08:22:13PM +0000, Sean Christopherson wrote:
> > i.e. ACPI_NUMA gets priority and thus amd_numa_init() will never be reached if
> > the NUMA topology is enumerated in the ACPI tables.  Furthermore, the VMM would
> > have to actually emulate an old AMD northbridge, which is also extremely unlikely.
> > 
> > The odds of breaking a guest are further diminised given that KVM doesn't emulate
> > the xAPIC ID => x2APIC ID hilarity on AMD CPUs and no one has complained.
> > 
> > So, rather than tie this to IPI virtualization, I think we should either make
> > the xAPIC ID read-only across the board,
> 
> We will go this way and defer the introduction of "xapic_id_writable" to the
> emergence of the "crazy" use case.
> 
> Levitsky, we plan to revise your patch 13 "[PATCH RESEND 13/30] KVM: x86: lapic:
> don't allow to change APIC ID when apic acceleration is enabled" to make xAPIC
> ID read-only regardless of APICv/AVIC and include it into IPI virtualization
> series (to eliminate the dependency on your AVIC series). Is it fine with you?


Absolutely!
> And does this patch 13 depend on other patches in your fixes?

This patch doesn't depend on anything.

There is also patch 14 in this series which closes a case where malicious userspace
could upload non default _x2apic id_. I  haven't yet written a unit test
to demonstrate this, but I will soon.

You don't need that patch for now IMHO.

> 
> > or if we want to hedge in case someone
> > has a crazy use case, make the xAPIC ID read-only by default, add a module param
> > to let userspace opt-in to a writable xAPIC ID, and report x2APIC and APICv as
> > unsupported if the xAPIC ID is writable.  E.g. rougly this, plus your AVIC patches
> > if we want to hedge.


Best regards,
	Maxim Levitsky