Re: [RFC PATCH 16/27] KVM: arm64: Prepare Hyp memory protection

[Will Deacon <will@kernel.org>]

Hi Quentin,

On Tue, Nov 17, 2020 at 06:15:56PM +0000, Quentin Perret wrote:
> When memory protection is enabled, the Hyp code needs the ability to
> create and manage its own page-table. To do so, introduce a new set of
> hypercalls to initialize Hyp memory protection.
> 
> During the init hcall, the hypervisor runs with the host-provided
> page-table and uses the trivial early page allocator to create its own
> set of page-tables, using a memory pool that was donated by the host.
> Specifically, the hypervisor creates its own mappings for __hyp_text,
> the Hyp memory pool, the __hyp_bss, the portion of hyp_vmemmap
> corresponding to the Hyp pool, among other things. It then jumps back in
> the idmap page, switches to use the newly-created pgd (instead of the
> temporary one provided by the host) and then installs the full-fledged
> buddy allocator which will then be the only one in used from then on.
> 
> Note that for the sake of symplifying the review, this only introduces
> the code doing this operation, without actually being called by anyhing
> yet. This will be done in a subsequent patch, which will introduce the
> necessary host kernel changes.

[...]

> diff --git a/arch/arm64/kvm/hyp/reserved_mem.c b/arch/arm64/kvm/hyp/reserved_mem.c
> new file mode 100644
> index 000000000000..02b0b18006f5
> --- /dev/null
> +++ b/arch/arm64/kvm/hyp/reserved_mem.c

[...]

> +extern bool enable_protected_kvm;
> +void __init reserve_kvm_hyp(void)
> +{
> +	u64 nr_pages, prev;
> +
> +	if (!enable_protected_kvm)
> +		return;
> +
> +	if (!is_hyp_mode_available() || is_kernel_in_hyp_mode())
> +		return;
> +
> +	if (kvm_nvhe_sym(hyp_memblock_nr) <= 0)
> +		return;
> +
> +	hyp_mem_size += num_possible_cpus() << PAGE_SHIFT;
> +	hyp_mem_size += hyp_s1_pgtable_size();
> +
> +	/*
> +	 * The hyp_vmemmap needs to be backed by pages, but these pages
> +	 * themselves need to be present in the vmemmap, so compute the number
> +	 * of pages needed by looking for a fixed point.
> +	 */
> +	nr_pages = 0;
> +	do {
> +		prev = nr_pages;
> +		nr_pages = (hyp_mem_size >> PAGE_SHIFT) + prev;
> +		nr_pages = DIV_ROUND_UP(nr_pages * sizeof(struct hyp_page), PAGE_SIZE);
> +		nr_pages += __hyp_pgtable_max_pages(nr_pages);
> +	} while (nr_pages != prev);
> +	hyp_mem_size += nr_pages << PAGE_SHIFT;
> +
> +	hyp_mem_base = memblock_find_in_range(0, memblock_end_of_DRAM(),
> +					      hyp_mem_size, SZ_2M);
> +	if (!hyp_mem_base) {
> +		kvm_err("Failed to reserve hyp memory\n");
> +		return;
> +	}
> +	memblock_reserve(hyp_mem_base, hyp_mem_size);

Why not use the RESERVEDMEM_OF_DECLARE() interface for the hypervisor
memory? That way, the hypervisor memory can either be statically partitioned
as a carveout or allocated dynamically for us -- we wouldn't need to care.

Will