linux-kernel - Re: [RFC PATCH v12 11/33] KVM: Introduce per-page memory attributes

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Date:   Mon, 25 Sep 2023 10:37:42 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v12 11/33] KVM: Introduce per-page memory attributes

On Thu, Sep 21, 2023, Yan Zhao wrote:
> On Wed, Sep 20, 2023 at 02:00:22PM -0700, Sean Christopherson wrote:
> > On Fri, Sep 15, 2023, Yan Zhao wrote:
> > > On Wed, Sep 13, 2023 at 06:55:09PM -0700, Sean Christopherson wrote:
> > > > +/* Set @attributes for the gfn range [@start, @end). */
> > > > +static int kvm_vm_set_mem_attributes(struct kvm *kvm, gfn_t start, gfn_t end,
> > > > +				     unsigned long attributes)
> > > > +{
> > > > +	struct kvm_mmu_notifier_range pre_set_range = {
> > > > +		.start = start,
> > > > +		.end = end,
> > > > +		.handler = kvm_arch_pre_set_memory_attributes,
> > > > +		.on_lock = kvm_mmu_invalidate_begin,
> > > > +		.flush_on_ret = true,
> > > > +		.may_block = true,
> > > > +	};
> > > > +	struct kvm_mmu_notifier_range post_set_range = {
> > > > +		.start = start,
> > > > +		.end = end,
> > > > +		.arg.attributes = attributes,
> > > > +		.handler = kvm_arch_post_set_memory_attributes,
> > > > +		.on_lock = kvm_mmu_invalidate_end,
> > > > +		.may_block = true,
> > > > +	};
> > > > +	unsigned long i;
> > > > +	void *entry;
> > > > +	int r = 0;
> > > > +
> > > > +	entry = attributes ? xa_mk_value(attributes) : NULL;
> > > Also here, do we need to get existing attributes of a GFN first ?
> > 
> > No?  @entry is the new value that will be set for all entries.  This line doesn't
> > touch the xarray in any way.  Maybe I'm just not understanding your question.
> Hmm, I thought this interface was to allow users to add/remove an attribute to a GFN
> rather than overwrite all attributes of a GFN. Now I think I misunderstood the intention.
> 
> But I wonder if there is a way for users to just add one attribute, as I don't find
> ioctl like KVM_GET_MEMORY_ATTRIBUTES for users to get current attributes and then to
> add/remove one based on that. e.g. maybe in future, KVM wants to add one attribute in
> kernel without being told by userspace ?

The plan is that memory attributes will be 100% userspace driven, i.e. that KVM
will never add its own attributes.  That's why there is (currently) no
KVM_GET_MEMORY_ATTRIBUTES, the intended usage model is that userspace is fully
responsible for managing attributes, and so should never need to query information
that it already knows.  If there's a compelling case for getting attributes then
we could certainly add such an ioctl(), but I hope we never need to add a GET
because that likely means we've made mistakes along the way.

Giving userspace full control of attributes allows for a simpler uAPI, e.g. if
userspace doesn't have full control, then setting or clearing bits requires a RMW
operation, which means creating a more complex ioctl().  That's why its a straight
SET operation and not an OR type operation.