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Message-ID: <Yt6/9V0S9of7dueW@google.com>
Date: Mon, 25 Jul 2022 16:08:21 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Jani Nikula <jani.nikula@...ux.intel.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
Zhenyu Wang <zhenyuw@...ux.intel.com>,
Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
Tom Lendacky <thomas.lendacky@....com>,
Ingo Molnar <mingo@...hat.com>,
David Airlie <airlied@...ux.ie>,
Thomas Gleixner <tglx@...utronix.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
intel-gfx@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>,
Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
Zhi Wang <zhi.a.wang@...el.com>,
Brijesh Singh <brijesh.singh@....com>,
"H. Peter Anvin" <hpa@...or.com>,
intel-gvt-dev@...ts.freedesktop.org,
dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write
tracking externally
On Wed, Jul 20, 2022, Maxim Levitsky wrote:
> On Sun, 2022-05-22 at 13:22 +0300, Maxim Levitsky wrote:
> > On Thu, 2022-05-19 at 16:37 +0000, Sean Christopherson wrote:
> > > On Wed, Apr 27, 2022, Maxim Levitsky wrote:
> > > > @@ -5753,6 +5752,10 @@ int kvm_mmu_init_vm(struct kvm *kvm)
> Now for nested AVIC, this is what I would like to do:
>
> - just like mmu, I prefer to register the write tracking notifier, when the
> VM is created.
>
> - just like mmu, write tracking should only be enabled when nested AVIC is
> actually used first time, so that write tracking is not always enabled when
> you just boot a VM with nested avic supported, since the VM might not use
> nested at all.
>
> Thus I either need to use the __kvm_page_track_register_notifier too for AVIC
> (and thus need to export it) or I need to have a boolean
> (nested_avic_was_used_once) and register the write tracking notifier only
> when false and do it not on VM creation but on first attempt to use nested
> AVIC.
>
> Do you think this is worth it? I mean there is some value of registering the
> notifier only when needed (this way it is not called for nothing) but it does
> complicate things a bit.
Compared to everything else that you're doing in the nested AVIC code, refcounting
the shared kvm_page_track_notifier_node object is a trivial amount of complexity.
And on that topic, do you have performance numbers to justify using a single
shared node? E.g. if every table instance has its own notifier, then no additional
refcounting is needed. It's not obvious that a shared node will provide better
performance, e.g. if there are only a handful of AVIC tables being shadowed, then
a linear walk of all nodes is likely fast enough, and doesn't bring the risk of
a write potentially being stalled due to having to acquire a VM-scoped mutex.
> I can also stash this boolean (like 'bool registered;') into the 'struct
> kvm_page_track_notifier_node', and thus allow the
> kvm_page_track_register_notifier to be called more that once - then I can
> also get rid of __kvm_page_track_register_notifier.
No, allowing redundant registration without proper refcounting leads to pain,
e.g. X registers, Y registers, X unregisters, kaboom.
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