linux-kernel - Re: [PATCH v2 16/27] KVM: x86: Add a new page-track hook to handle memslot deletion

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Message-ID: <ZBSTapUu+VzWmIfQ@google.com>
Date:   Fri, 17 Mar 2023 09:20:58 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 16/27] KVM: x86: Add a new page-track hook to handle
 memslot deletion

On Fri, Mar 17, 2023, Yan Zhao wrote:
> On Fri, Mar 10, 2023 at 04:22:47PM -0800, Sean Christopherson wrote:
> > From: Yan Zhao <yan.y.zhao@...el.com>
> > 
> > Add a new page-track hook, track_remove_region(), that is called when a
> > memslot DELETE operation is about to be committed.  The "remove" hook
> > will be used by KVMGT and will effectively replace the existing
> > track_flush_slot() altogether now that KVM itself doesn't rely on the
> > "flush" hook either.
> > 
> > The "flush" hook is flawed as it's invoked before the memslot operation
> > is guaranteed to succeed, i.e. KVM might ultimately keep the existing
> > memslot without notifying external page track users, a.k.a. KVMGT.  In
> > practice, this can't currently happen on x86, but there are no guarantees
> > that won't change in the future, not to mention that "flush" does a very
> > poor job of describing what is happening.
> > 
> > Pass in the gfn+nr_pages instead of the slot itself so external users,
> > i.e. KVMGT, don't need to exposed to KVM internals (memslots).  This will
> > help set the stage for additional cleanups to the page-track APIs.
> > 
> > Cc: Zhenyu Wang <zhenyuw@...ux.intel.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> > Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ...
> 
> > +void kvm_page_track_delete_slot(struct kvm *kvm, struct kvm_memory_slot *slot)
> > +{
> > +	struct kvm_page_track_notifier_head *head;
> > +	struct kvm_page_track_notifier_node *n;
> > +	int idx;
> > +
> > +	head = &kvm->arch.track_notifier_head;
> > +
> > +	if (hlist_empty(&head->track_notifier_list))
> > +		return;
> > +
> > +	idx = srcu_read_lock(&head->track_srcu);
> > +	hlist_for_each_entry_srcu(n, &head->track_notifier_list, node,
> > +				srcu_read_lock_held(&head->track_srcu))
> Sorry, not sure why the alignment here is not right.
> Patchwork just sent me a mail to complain about it.
> Would you mind helping fix it in the next version?

Ah, it's off by two spaces, should be 

	hlist_for_each_entry_srcu(n, &head->track_notifier_list, node,
				  srcu_read_lock_held(&head->track_srcu))

I'll get it fixed in the next version.