Re: [RFC PATCH 12/15] KVM: x86/mmu: Split large pages when dirty logging is enabled

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 11/20/2021 5:27 AM, David Matlack wrote:
> When dirty logging is enabled without initially-all-set, attempt to
> split all large pages in the memslot down to 4KB pages so that vCPUs
> do not have to take expensive write-protection faults to split large
> pages.
> 
> Large page splitting is best-effort only. This commit only adds the
> support for the TDP MMU, and even there splitting may fail due to out
> of memory conditions. Failures to split a large page is fine from a
> correctness standpoint because we still always follow it up by write-
> protecting any remaining large pages.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>

> +int mmu_topup_split_caches(struct kvm *kvm)
> +{
> +	struct kvm_mmu_memory_caches *split_caches = &kvm->arch.split_caches;
> +	int r;
> +
> +	assert_split_caches_invariants(kvm);
> +
> +	r = kvm_mmu_topup_memory_cache(&split_caches->page_header_cache, 1);
> +	if (r)
> +		goto out;
> +
> +	r = kvm_mmu_topup_memory_cache(&split_caches->shadow_page_cache, 1);
> +	if (r)
> +		goto out;
> +
> +	return 0;
> +
> +out:
> +	pr_warn("Failed to top-up split caches. Will not split large pages.\n");
> +	return r;
> +}
> +
> +static void mmu_free_split_caches(struct kvm *kvm)
> +{
> +	assert_split_caches_invariants(kvm);
> +
> +	kvm_mmu_free_memory_cache(&kvm->arch.split_caches.pte_list_desc_cache);
                                                              ^^^^^^^^^^^^^^
I believe this should be page_header_cache.

> +	kvm_mmu_free_memory_cache(&kvm->arch.split_caches.shadow_page_cache);
> +}

Regards
Nikunj