Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally

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: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally
Date: Mon, 25 Jul 2022 16:08:21 +0000	[thread overview]
Message-ID: <Yt6/9V0S9of7dueW@google.com> (raw)
In-Reply-To: <c22a18631c2067871b9ed8a9246ad58fa1ab8947.camel@redhat.com>

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.