Subject: Re: [RFC PATCH v2 16/26] KVM: arm64: Prepare Hyp memory protection

From: Sean Christopherson <[email-masked]>
Date: Fri, 19 Feb 2021 10:32:58 -0800

On Wed, Feb 03, 2021, Will Deacon wrote:
> On Fri, Jan 08, 2021 at 12:15:14PM +0000, Quentin Perret wrote:

...

> > +static inline unsigned long hyp_s1_pgtable_size(void)
> > +{

...

> > +		res += nr_pages << PAGE_SHIFT;
> > +	}
> > +
> > +	/* Allow 1 GiB for private mappings */
> > +	nr_pages = (1 << 30) >> PAGE_SHIFT;
> 
> SZ_1G >> PAGE_SHIFT

Where does the 1gb magic number come from?  IIUC, this is calculating the number
of pages needed for the hypervisor's Stage-1 page tables.  The amount of memory
needed for those page tables should be easily calculated, and assuming huge
pages can be used, should be far less the 1gb.
 
> > +	nr_pages = __hyp_pgtable_max_pages(nr_pages);
> > +	res += nr_pages << PAGE_SHIFT;
> > +
> > +	return res;

...

> > +void __init kvm_hyp_reserve(void)
> > +{
> > +	u64 nr_pages, prev;
> > +
> > +	if (!is_hyp_mode_available() || is_kernel_in_hyp_mode())
> > +		return;
> > +
> > +	if (kvm_get_mode() != KVM_MODE_PROTECTED)
> > +		return;
> > +
> > +	if (kvm_nvhe_sym(hyp_memblock_nr) < 0) {
> > +		kvm_err("Failed to register hyp memblocks\n");
> > +		return;
> > +	}
> > +
> > +	sort_memblock_regions();
> > +
> > +	/*
> > +	 * We don't know the number of possible CPUs yet, so allocate for the
> > +	 * worst case.
> > +	 */
> > +	hyp_mem_size += NR_CPUS << PAGE_SHIFT;

Is this for per-cpu stack?

If so, what guarantees a single page is sufficient?  Mostly a curiosity question,
since it looks like this is an existing assumption by init_hyp_mode().  Shouldn't
the required stack size be defined in bytes and converted to pages, or is there a
guarantee that 64kb pages will be used?

> There was a recent patch bumping NR_CPUs to 512, so this would be 32MB
> with 64k pages. Is it possible to return memory to the host later on once
> we have a better handle on the number of CPUs in the system?

Does kvm_hyp_reserve() really need to be called during bootmem_init()?  What
prevents doing the reservation during init_hyp_mode()?  If the problem is that
pKVM needs a single contiguous chunk of memory, then it might be worth solving
_that_ problem, e.g. letting the host donate memory in N-byte chunks instead of
requiring a single huge blob of memory.
 
> > +	hyp_mem_size += hyp_s1_pgtable_size();

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