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Date:   Mon, 14 Aug 2023 14:54:32 -0700
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 <02239d95-0253-a223-28c2-016cca3ab4d2@linux.intel.com>
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Subject: Re: [RFC PATCH v11 08/29] KVM: Introduce per-page memory attributes
From:   Sean Christopherson <seanjc@google.com>
To:     Binbin Wu <binbin.wu@linux.intel.com>
Cc:     Chao Peng <chao.p.peng@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paul Moore <paul@paul-moore.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
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        Yu Zhang <yu.c.zhang@linux.intel.com>,
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        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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On Mon, Aug 14, 2023, Binbin Wu wrote:
> 
> On 7/19/2023 7:44 AM, Sean Christopherson wrote:
> > +	struct kvm_mmu_notifier_range post_set_range = {
> > +		.start = start,
> > +		.end = end,
> > +		.arg.attributes = attributes,
> > +		.handler = kvm_arch_post_set_memory_attributes,
> > +		.on_lock = (void *)kvm_null_fn,
> > +		.on_unlock = kvm_mmu_invalidate_end,
> > +		.may_block = true,
> > +	};
> > +	unsigned long i;
> > +	void *entry;
> > +	int r;
> > +
> > +	entry = attributes ? xa_mk_value(attributes) : NULL;
> Why attributes of value 0 is considered not a value? Is it because 0 is not
> a valid value when RWX is considered in the future?

0 values don't require an entry in the xarray, i.e. don't need to be stored and
so don't consume memory.  The potential conflict with a RWX=0 entry has already
been noted, but we'll cross that bridge when we get to it, e.g. KVM can easily
support RWX=0 by using an internal "valid" flag.

> Both the changelog and the document added mention that the address and size
> of attrs will be updated to
> "reflect the actual pages of the memory range have been successfully set to
> the attributes", but it doesn't.

Yeah, on the todo list, all of the changelogs are horribly stale.