On Wed, Oct 19, 2022, David Matlack wrote:
> Now that the TDP MMU has a mechanism to split huge pages, use it in the
> fault path when a huge page needs to be replaced with a mapping at a
> lower level.
>
> This change reduces the negative performance impact of NX HugePages.
> Prior to this change if a vCPU executed from a huge page and NX
> HugePages was enabled, the vCPU would take a fault, zap the huge page,
> and mapping the faulting address at 4KiB with execute permissions
> enabled. The rest of the memory would be left *unmapped* and have to be
> faulted back in by the guest upon access (read, write, or execute). If
> guest is backed by 1GiB, a single execute instruction can zap an entire
> GiB of its physical address space.
>
> For example, it can take a VM longer to execute from its memory than to
> populate that memory in the first place:
>
> $ ./execute_perf_test -s anonymous_hugetlb_1gb -v96
>
> Populating memory : 2.748378795s
> Executing from memory : 2.899670885s
>
> With this change, such faults split the huge page instead of zapping it,
> which avoids the non-present faults on the rest of the huge page:
>
> $ ./execute_perf_test -s anonymous_hugetlb_1gb -v96
>
> Populating memory : 2.729544474s
> Executing from memory : 0.111965688s <---
>
> This change also reduces the performance impact of dirty logging when
> eager_page_split=N. eager_page_split=N (abbreviated "eps=N" below) can
> be desirable for read-heavy workloads, as it avoids allocating memory to
> split huge pages that are never written and avoids increasing the TLB
> miss cost on reads of those pages.
>
> | Config: ept=Y, tdp_mmu=Y, 5% writes |
> | Iteration 1 dirty memory time |
> | --------------------------------------------- |
> vCPU Count | eps=N (Before) | eps=N (After) | eps=Y |
> ------------ | -------------- | ------------- | ------------ |
> 2 | 0.332305091s | 0.019615027s | 0.006108211s |
> 4 | 0.353096020s | 0.019452131s | 0.006214670s |
> 8 | 0.453938562s | 0.019748246s | 0.006610997s |
> 16 | 0.719095024s | 0.019972171s | 0.007757889s |
> 32 | 1.698727124s | 0.021361615s | 0.012274432s |
> 64 | 2.630673582s | 0.031122014s | 0.016994683s |
> 96 | 3.016535213s | 0.062608739s | 0.044760838s |
>
> Eager page splitting remains beneficial for write-heavy workloads, but
> the gap is now reduced.
>
> | Config: ept=Y, tdp_mmu=Y, 100% writes |
> | Iteration 1 dirty memory time |
> | --------------------------------------------- |
> vCPU Count | eps=N (Before) | eps=N (After) | eps=Y |
> ------------ | -------------- | ------------- | ------------ |
> 2 | 0.317710329s | 0.296204596s | 0.058689782s |
> 4 | 0.337102375s | 0.299841017s | 0.060343076s |
> 8 | 0.386025681s | 0.297274460s | 0.060399702s |
> 16 | 0.791462524s | 0.298942578s | 0.062508699s |
> 32 | 1.719646014s | 0.313101996s | 0.075984855s |
> 64 | 2.527973150s | 0.455779206s | 0.079789363s |
> 96 | 2.681123208s | 0.673778787s | 0.165386739s |
>
> Further study is needed to determine if the remaining gap is acceptable
> for customer workloads or if eager_page_split=N still requires a-priori
> knowledge of the VM workload, especially when considering these costs
> extrapolated out to large VMs with e.g. 416 vCPUs and 12TB RAM.
>
> Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 72 ++++++++++++++++++--------------------
> 1 file changed, 34 insertions(+), 38 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 4e5b3ae824c1..c53767104d5b 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1146,6 +1146,9 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
> return 0;
> }
>
> +static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
> + struct kvm_mmu_page *sp, bool shared);
> +
> /*
> * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing
> * page tables and SPTEs to translate the faulting guest physical address.
> @@ -1171,49 +1174,42 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> if (iter.level == fault->goal_level)
> break;
>
> - /*
> - * If there is an SPTE mapping a large page at a higher level
> - * than the target, that SPTE must be cleared and replaced
> - * with a non-leaf SPTE.
> - */
> + /* Step down into the lower level page table if it exists. */
> if (is_shadow_present_pte(iter.old_spte) &&
> - is_large_pte(iter.old_spte)) {
> - if (tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter))
> - break;
> + !is_large_pte(iter.old_spte))
> + continue;
>
> - /*
> - * The iter must explicitly re-read the spte here
> - * because the new value informs the !present
> - * path below.
> - */
> - iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep);
> - }
> + /*
> + * If SPTE has been frozen by another thread, just give up and
> + * retry, avoiding unnecessary page table allocation and free.
> + */
> + if (is_removed_spte(iter.old_spte))
> + break;
>
> - if (!is_shadow_present_pte(iter.old_spte)) {
> - /*
> - * If SPTE has been frozen by another thread, just
> - * give up and retry, avoiding unnecessary page table
> - * allocation and free.
> - */
> - if (is_removed_spte(iter.old_spte))
> - break;
> + /*
> + * The SPTE is either non-present or points to a huge page that
> + * needs to be split.
> + */
> + sp = tdp_mmu_alloc_sp(vcpu);
> + tdp_mmu_init_child_sp(sp, &iter);
>
> - sp = tdp_mmu_alloc_sp(vcpu);
> - tdp_mmu_init_child_sp(sp, &iter);
> + sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
>
> - sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
> + if (is_shadow_present_pte(iter.old_spte))
> + ret = tdp_mmu_split_huge_page(kvm, &iter, sp, true);
> + else
> + ret = tdp_mmu_link_sp(kvm, &iter, sp, true);
>
> - if (tdp_mmu_link_sp(kvm, &iter, sp, true)) {
> - tdp_mmu_free_sp(sp);
> - break;
> - }
> + if (ret) {
> + tdp_mmu_free_sp(sp);
> + break;
> + }
>
> - if (fault->huge_page_disallowed &&
> - fault->req_level >= iter.level) {
> - spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> - track_possible_nx_huge_page(kvm, sp);
> - spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> - }
> + if (fault->huge_page_disallowed &&
> + fault->req_level >= iter.level) {
> + spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> + track_possible_nx_huge_page(kvm, sp);
> + spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> }
> }
>
> @@ -1484,8 +1480,6 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
> const int level = iter->level;
> int ret, i;
>
> - tdp_mmu_init_child_sp(sp, iter);
> -
David, thanks for the alignment with the precise nx huge page series.
Nit: since this patch puts tdp_mmu_init_child_sp() out of the
tdp_mmu_split_huge_page(), can we add a comment mentioned that, i.e.,
initialization of child sp is required before invoking the function?
With that action done,
Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> /*
> * No need for atomics when writing to sp->spt since the page table has
> * not been linked in yet and thus is not reachable from any other CPU.
> @@ -1561,6 +1555,8 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
> continue;
> }
>
> + tdp_mmu_init_child_sp(sp, &iter);
> +
> if (tdp_mmu_split_huge_page(kvm, &iter, sp, shared))
> goto retry;
>
> --
> 2.38.0.413.g74048e4d9e-goog
>
