Re: [PATCH v2 06/18] KVM: x86/mmu: do not consult levels when freeing roots

Maxim Levitsky
2022-02-23T14:59:37Z

On Thu, 2022-02-17 at 16:03 -0500, Paolo Bonzini wrote:
> Right now, PGD caching requires a complicated dance of first computing
> the MMU role and passing it to __kvm_mmu_new_pgd(), and then separately calling
> kvm_init_mmu().
>
> Part of this is due to kvm_mmu_free_roots using mmu->root_level and
> mmu->shadow_root_level to distinguish whether the page table uses a single
> root or 4 PAE roots. Because kvm_init_mmu() can overwrite mmu->root_level,
> kvm_mmu_free_roots() must be called before kvm_init_mmu().
>
> However, even after kvm_init_mmu() there is a way to detect whether the
> page table may hold PAE roots, as root.hpa isn't backed by a shadow when
> it points at PAE roots. Using this method results in simpler code, and
> is one less obstacle in moving all calls to __kvm_mmu_new_pgd() after the
> MMU has been initialized.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index a478667d7561..e1578f71feae 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3240,12 +3240,15 @@ void kvm_mmu_free_roots(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> struct kvm *kvm = vcpu->kvm;
> int i;
> LIST_HEAD(invalid_list);
> - bool free_active_root = roots_to_free & KVM_MMU_ROOT_CURRENT;
> + bool free_active_root;
>
> BUILD_BUG_ON(KVM_MMU_NUM_PREV_ROOTS >= BITS_PER_LONG);
>
> /* Before acquiring the MMU lock, see if we need to do any real work. */
> - if (!(free_active_root && VALID_PAGE(mmu->root.hpa))) {
> + free_active_root = (roots_to_free & KVM_MMU_ROOT_CURRENT)
> + && VALID_PAGE(mmu->root.hpa);
> +
> + if (!free_active_root) {
> for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
> if ((roots_to_free & KVM_MMU_ROOT_PREVIOUS(i)) &&
> VALID_PAGE(mmu->prev_roots[i].hpa))
> @@ -3263,8 +3266,7 @@ void kvm_mmu_free_roots(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> &invalid_list);
>
> if (free_active_root) {
> - if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
> - (mmu->root_level >= PT64_ROOT_4LEVEL || mmu->direct_map)) {
> + if (to_shadow_page(mmu->root.hpa)) {
> mmu_free_root_page(kvm, &mmu->root.hpa, &invalid_list);
> } else if (mmu->pae_root) {
> for (i = 0; i < 4; ++i) {

Makes sense, although that will collide hard with that RFC of backing all shadow pages
with kvm mmu pages.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky