On Mon, Dec 13, 2021 at 10:59:14PM +0000, David Matlack wrote:
> When dirty logging is enabled without initially-all-set, attempt to
> split all huge pages in the memslot down to 4KB pages so that vCPUs
> do not have to take expensive write-protection faults to split huge
> pages.
>
> Huge 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 huge page is fine from a
> correctness standpoint because we still always follow it up by write-
> protecting any remaining huge pages.
>
> Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
Thanks for adding the knob.
Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
One trivial nitpick below:
> +u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index, unsigned int access)
> +{
> + u64 child_spte;
> + int child_level;
> +
> + if (WARN_ON(is_mmio_spte(huge_spte)))
> + return 0;
> +
> + if (WARN_ON(!is_shadow_present_pte(huge_spte)))
> + return 0;
> +
> + if (WARN_ON(!is_large_pte(huge_spte)))
> + return 0;
> +
> + child_spte = huge_spte;
> + child_level = huge_level - 1;
> +
> + /*
> + * The child_spte already has the base address of the huge page being
> + * split. So we just have to OR in the offset to the page at the next
> + * lower level for the given index.
> + */
> + child_spte |= (index * KVM_PAGES_PER_HPAGE(child_level)) << PAGE_SHIFT;
> +
> + if (child_level == PG_LEVEL_4K) {
> + child_spte &= ~PT_PAGE_SIZE_MASK;
> +
> + /* Allow execution for 4K pages if it was disabled for NX HugePages. */
> + if (is_nx_huge_page_enabled() && access & ACC_EXEC_MASK)
IMHO clearer to use brackets ("A && (B & C)").
I don't even see anywhere that the tdp mmu disables the EXEC bit for 4K.. if
that's true then perhaps we can even drop "access" and this check? But I could
have missed something.
> + child_spte = mark_spte_executable(child_spte);
> + }
> +
> + return child_spte;
> +}
--
Peter Xu
