linux-kernel - Re: [PATCH v5 7/8] KVM: VMX: Update PID-pointer table entry when APIC ID is changed

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Date:   Wed, 23 Feb 2022 12:26:31 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Chao Gao <chao.gao@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Zeng Guang <guang.zeng@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        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" <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Luck, Tony" <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>,
        "Huang, Kai" <kai.huang@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hu, Robert" <robert.hu@...el.com>
Subject: Re: [PATCH v5 7/8] KVM: VMX: Update PID-pointer table entry when
 APIC ID is changed

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