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

From: Sean Christopherson <[email-masked]>
Date: Wed, 20 Sep 2023 14:00:22 -0700

On Fri, Sep 15, 2023, Yan Zhao wrote:
> On Wed, Sep 13, 2023 at 06:55:09PM -0700, Sean Christopherson wrote:
> > From: Chao Peng <[email-masked]>
> > 
> > In confidential computing usages, whether a page is private or shared is
> > necessary information for KVM to perform operations like page fault
> > handling, page zapping etc. There are other potential use cases for
> > per-page memory attributes, e.g. to make memory read-only (or no-exec,
> > or exec-only, etc.) without having to modify memslots.
> > 
> ...
> >> +bool kvm_range_has_memory_attributes(struct kvm *kvm, gfn_t start, gfn_t end,
> > +				     unsigned long attrs)
> > +{
> > +	XA_STATE(xas, &kvm->mem_attr_array, start);
> > +	unsigned long index;
> > +	bool has_attrs;
> > +	void *entry;
> > +
> > +	rcu_read_lock();
> > +
> > +	if (!attrs) {
> > +		has_attrs = !xas_find(&xas, end);
> > +		goto out;
> > +	}
> > +
> > +	has_attrs = true;
> > +	for (index = start; index < end; index++) {
> > +		do {
> > +			entry = xas_next(&xas);
> > +		} while (xas_retry(&xas, entry));
> > +
> > +		if (xas.xa_index != index || xa_to_value(entry) != attrs) {
> Should "xa_to_value(entry) != attrs" be "!(xa_to_value(entry) & attrs)" ?

No, the exact comparsion is deliberate.  The intent of the API is to determine
if the entire range already has the desired attributes, not if there is overlap
between the two.

E.g. if/when RWX attributes are supported, the exact comparison is needed to
handle a RW => R conversion.

> > +			has_attrs = false;
> > +			break;
> > +		}
> > +	}
> > +
> > +out:
> > +	rcu_read_unlock();
> > +	return has_attrs;
> > +}
> > +
> ...
> > +/* 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.