On Mon, Dec 13, 2021, David Matlack wrote:
> Factor out the logic to atomically replace an SPTE with an SPTE that
> points to a new page table. This will be used in a follow-up commit to
> split a large page SPTE into one level lower.
>
> Opportunistically drop the kvm_mmu_get_page tracepoint in
> kvm_tdp_mmu_map() since it is redundant with the identical tracepoint in
> alloc_tdp_mmu_page().
>
> Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 48 +++++++++++++++++++++++++++-----------
> 1 file changed, 34 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 656ebf5b20dc..dbd07c10d11a 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -950,6 +950,36 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
> return ret;
> }
>
> +/*
> + * tdp_mmu_install_sp_atomic - Atomically replace the given spte with an
> + * spte pointing to the provided page table.
> + *
> + * @kvm: kvm instance
> + * @iter: a tdp_iter instance currently on the SPTE that should be set
> + * @sp: The new TDP page table to install.
> + * @account_nx: True if this page table is being installed to split a
> + * non-executable huge page.
> + *
> + * Returns: True if the new page table was installed. False if spte being
> + * replaced changed, causing the atomic compare-exchange to fail.
I'd prefer to return an int with 0/-EBUSY on success/fail. Ditto for the existing
tdp_mmu_set_spte_atomic(). Actually, if you add a prep patch to make that happen,
then this can be:
u64 spte = make_nonleaf_spte(sp->spt, !shadow_accessed_mask);
int ret;
ret = tdp_mmu_set_spte_atomic(kvm, iter, spte);
if (ret)
return ret;
tdp_mmu_link_page(kvm, sp, account_nx);
return 0;
> + * If this function returns false the sp will be freed before
> + * returning.
Uh, no it's not? The call to tdp_mmu_free_sp() is still done by kvm_tdp_mmu_map().
> + */
> +static bool tdp_mmu_install_sp_atomic(struct kvm *kvm,
Hmm, so this helper is the only user of tdp_mmu_link_page(), and _that_ helper
is rather tiny. And this would also be a good opportunity to clean up the
"(un)link_page" verbiage, as the bare "page" doesn't communicate to the reader
that it's for linking shadow pages, e.g. not struct page.
So, what about folding in tdp_mmu_link_page(), naming this helper either
tdp_mmu_link_sp_atomic() or tdp_mmu_link_shadow_page_atomic(), and then renaming
tdp_mmu_unlink_page() accordingly? And for bonus points, add a blurb in the
function comment like:
* Note the lack of a non-atomic variant! The TDP MMU always builds its
* page tables while holding mmu_lock for read.
> + struct tdp_iter *iter,
> + struct kvm_mmu_page *sp,
> + bool account_nx)
> +{
> + u64 spte = make_nonleaf_spte(sp->spt, !shadow_accessed_mask);
> +
> + if (!tdp_mmu_set_spte_atomic(kvm, iter, spte))
> + return false;
> +
> + tdp_mmu_link_page(kvm, sp, account_nx);
> +
> + return true;
> +}
> +
> /*
> * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing
> * page tables and SPTEs to translate the faulting guest physical address.
> @@ -959,8 +989,6 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> struct kvm_mmu *mmu = vcpu->arch.mmu;
> struct tdp_iter iter;
> struct kvm_mmu_page *sp;
> - u64 *child_pt;
> - u64 new_spte;
> int ret;
>
> kvm_mmu_hugepage_adjust(vcpu, fault);
> @@ -996,6 +1024,9 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> }
>
> if (!is_shadow_present_pte(iter.old_spte)) {
> + bool account_nx = fault->huge_page_disallowed &&
> + fault->req_level >= iter.level;
> +
> /*
> * If SPTE has been frozen by another thread, just
> * give up and retry, avoiding unnecessary page table
> @@ -1005,18 +1036,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> break;
>
> sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level - 1);
> - child_pt = sp->spt;
> -
> - new_spte = make_nonleaf_spte(child_pt,
> - !shadow_accessed_mask);
> -
> - if (tdp_mmu_set_spte_atomic(vcpu->kvm, &iter, new_spte)) {
> - tdp_mmu_link_page(vcpu->kvm, sp,
> - fault->huge_page_disallowed &&
> - fault->req_level >= iter.level);
> -
> - trace_kvm_mmu_get_page(sp, true);
> - } else {
> + if (!tdp_mmu_install_sp_atomic(vcpu->kvm, &iter, sp, account_nx)) {
> tdp_mmu_free_sp(sp);
> break;
> }
> --
> 2.34.1.173.g76aa8bc2d0-goog
>
