linux-kernel - Re: [PATCH v13 13/35] KVM: Introduce per-page memory attributes

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Date:   Thu, 2 Nov 2023 10:55:31 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH v13 13/35] KVM: Introduce per-page memory attributes

On Thu, 2023-11-02 at 11:32 +0100, Paolo Bonzini wrote:
> > > +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> > > +static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
> > > +{
> > > +	return xa_to_value(xa_load(&kvm->mem_attr_array, gfn));
> > > +}
> > 
> > Only call xa_to_value() when xa_load() returns !NULL?
> 
> This xarray does not store a pointer, therefore xa_load() actually 
> returns an integer that is tagged with 1 in the low bit:
> 
> static inline unsigned long xa_to_value(const void *entry)
> {
>          return (unsigned long)entry >> 1;
> }
> 
> Returning zero for an empty entry is okay, so the result of xa_load() 
> can be used directly.

Thanks for explaining.  I was thinking perhaps it's better to do:

	void *entry = xa_load(...);
	
	return xa_is_value(entry) ? xa_to_value(entry) : 0;

But "NULL (0) >> 1" is still 0, so yes we can use directly.