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From: Sean Christopherson <seanjc@google.com>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm@vger.kernel.org, Wanpeng Li <wanpengli@tencent.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@intel.com>,
	Zhenyu Wang <zhenyuw@linux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Ingo Molnar <mingo@redhat.com>, David Airlie <airlied@linux.ie>,
	Thomas Gleixner <tglx@linutronix.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, intel-gfx@lists.freedesktop.org,
	Daniel Vetter <daniel@ffwll.ch>, Borislav Petkov <bp@alien8.de>,
	Joerg Roedel <joro@8bytes.org>,
	linux-kernel@vger.kernel.org, Jim Mattson <jmattson@google.com>,
	Zhi Wang <zhi.a.wang@intel.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	intel-gvt-dev@lists.freedesktop.org,
	dri-devel@lists.freedesktop.org
Subject: Re: Nested AVIC design (was:Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally)
Date: Thu, 29 Sep 2022 22:38:04 +0000	[thread overview]
Message-ID: <YzYeTCsNfQWccKJ9@google.com> (raw)
In-Reply-To: <fe76ea902a38a10e2d8078fd9e5a71a0c7724d84.camel@redhat.com>

On Mon, Aug 08, 2022, Maxim Levitsky wrote:
> Hi Sean, Paolo, and everyone else who wants to review my nested AVIC work.

Before we dive deep into design details, I think we should first decide whether
or not nested AVIC is worth pursing/supporting.

  - Rome has a ucode/silicon bug with no known workaround and no anticipated fix[*];
    AMD's recommended "workaround" is to disable AVIC.
  - AVIC is not available in Milan, which may or may not be related to the
    aforementioned bug.
  - AVIC is making a comeback on Zen4, but Zen4 comes with x2AVIC.
  - x2APIC is likely going to become ubiquitous, e.g. Intel is effectively
    requiring x2APIC to fudge around xAPIC bugs.
  - It's actually quite realistic to effectively force the guest to use x2APIC,
    at least if it's a Linux guest.  E.g. turn x2APIC on in BIOS, which is often
    (always?) controlled by the host, and Linux will use x2APIC.

In other words, given that AVIC is well on its way to becoming a "legacy" feature,
IMO there needs to be a fairly strong use case to justify taking on this much code
and complexity.  ~1500 lines of code to support a feature that has historically
been buggy _without_ nested support is going to require a non-trivial amount of
effort to review, stabilize, and maintain.

[*] 1235 "Guest With AVIC (Advanced Virtual Interrupt Controller) Enabled May Fail
    to Process IPI (Inter-Processor Interrupt) Until Guest Is Re-Scheduled" in
    https://www.amd.com/system/files/TechDocs/56323-PUB_1.00.pdf

  reply	other threads:[~2022-09-29 22:44 UTC|newest]

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-27 20:02 [RFC PATCH v3 00/19] RFC: nested AVIC Maxim Levitsky
2022-04-27 20:02 ` [RFC PATCH v3 01/19] KVM: x86: document AVIC/APICv inhibit reasons Maxim Levitsky
2022-05-18 15:56   ` Sean Christopherson
2022-05-18 17:13     ` Maxim Levitsky
2022-04-27 20:02 ` [RFC PATCH v3 02/19] KVM: x86: inhibit APICv/AVIC when the guest and/or host changes apic id/base from the defaults Maxim Levitsky
2022-05-18  8:28   ` Chao Gao
2022-05-18  9:50     ` Maxim Levitsky
2022-05-18 11:51       ` Chao Gao
2022-05-18 12:36         ` Maxim Levitsky
2022-05-18 15:39       ` Sean Christopherson
2022-05-18 17:15         ` Maxim Levitsky
2022-05-19 16:06   ` Sean Christopherson
2022-05-22  9:03     ` Maxim Levitsky
2022-05-22 14:47       ` Jim Mattson
2022-05-23  6:50         ` Maxim Levitsky
2022-05-23 17:22           ` Jim Mattson
2022-05-23 17:31           ` Sean Christopherson
2022-06-23  9:44     ` Maxim Levitsky
2022-04-27 20:02 ` [RFC PATCH v3 03/19] KVM: x86: SVM: remove avic's broken code that updated APIC ID Maxim Levitsky
2022-05-19 16:10   ` Sean Christopherson
2022-05-22  9:01     ` Maxim Levitsky
2022-05-23 17:19       ` Sean Christopherson
2022-04-27 20:02 ` [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally Maxim Levitsky
2022-05-19 16:27   ` Sean Christopherson
2022-05-22 10:21     ` Maxim Levitsky
2022-05-19 16:37   ` Sean Christopherson
2022-05-22 10:22     ` Maxim Levitsky
2022-07-20 14:42       ` Maxim Levitsky
2022-07-25 16:08         ` Sean Christopherson
2022-07-28  7:46           ` Maxim Levitsky
2022-08-01 15:53             ` Maxim Levitsky
2022-08-01 17:20             ` Sean Christopherson
2022-08-08 13:13               ` Nested AVIC design (was:Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally) Maxim Levitsky
2022-09-29 22:38                 ` Sean Christopherson [this message]
2022-10-03  7:27                   ` Maxim Levitsky
2022-11-10  0:47                     ` Sean Christopherson
2022-04-27 20:03 ` [RFC PATCH v3 05/19] x86: KVMGT: use kvm_page_track_write_tracking_enable Maxim Levitsky
2022-05-19 16:38   ` Sean Christopherson
2022-04-27 20:03 ` [RFC PATCH v3 06/19] KVM: x86: mmu: add gfn_in_memslot helper Maxim Levitsky
2022-05-19 16:43   ` Sean Christopherson
2022-05-22 10:22     ` Maxim Levitsky
2022-05-22 12:12     ` Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 07/19] KVM: x86: mmu: tweak fast path for emulation of access to nested NPT pages Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 08/19] KVM: x86: SVM: move avic state to separate struct Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 09/19] KVM: x86: nSVM: add nested AVIC tracepoints Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 10/19] KVM: x86: nSVM: implement AVIC's physid/logid table access helpers Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 11/19] KVM: x86: nSVM: implement shadowing of AVIC's physical id table Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 12/19] KVM: x86: nSVM: make nested AVIC physid write tracking be aware of the host scheduling Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 13/19] KVM: x86: nSVM: wire nested AVIC to nested guest entry/exit Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 14/19] KVM: x86: rename .set_apic_access_page_addr to reload_apic_access_page Maxim Levitsky
2022-05-19 16:55   ` Sean Christopherson
2022-05-22 10:22     ` Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 15/19] KVM: x86: nSVM: add code to reload AVIC physid table when it is invalidated Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 16/19] KVM: x86: nSVM: implement support for nested AVIC vmexits Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 17/19] KVM: x86: nSVM: implement nested AVIC doorbell emulation Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 18/19] KVM: x86: SVM/nSVM: add optional non strict AVIC doorbell mode Maxim Levitsky
2022-04-27 20:03 ` [RFC PATCH v3 19/19] KVM: x86: nSVM: expose the nested AVIC to the guest Maxim Levitsky

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