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>
Subject: Re: [PATCH v5 7/8] KVM: VMX: Update PID-pointer table entry when APIC ID is changed
Date: Wed, 23 Feb 2022 12:26:31 +0200	[thread overview]
Message-ID: <7e7d16f2919f4bc708a0da3237161b4325a867c5.camel@redhat.com> (raw)
In-Reply-To: <20220223061037.GA21263@gao-cwp>

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