On Wed, Nov 24, 2021 at 09:44:20PM +0000, Mingwei Zhang wrote:
> Not stepping down in TDP iterator in `zap_all` case avoids re-reading the
> non-leaf SPTEs, thus accelerates the zapping process . But when the number
> of SPTEs is too large, we may run out of CPU time and causes a RCU stall
> warnings in __handle_changed_pte() in the context of zap_gfn_range().
>
> Revert this patch to allow eliminating RCU stall warning using a two-phase
> zapping for `zap_all` case.
>
> This reverts commit 0103098fb4f13b447b26ed514bcd3140f6791047.
>
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> Cc: Ben Gardon <bgardon@xxxxxxxxxx>
> Cc: David Matlack <dmatlack@xxxxxxxxxx>
>
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 9 +--------
> 1 file changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 7c5dd83e52de..89d16bb104de 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -706,12 +706,6 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> bool zap_all = (start == 0 && end >= max_gfn_host);
> struct tdp_iter iter;
>
> - /*
> - * No need to try to step down in the iterator when zapping all SPTEs,
> - * zapping the top-level non-leaf SPTEs will recurse on their children.
> - */
> - int min_level = zap_all ? root->role.level : PG_LEVEL_4K;
> -
> /*
> * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
> * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
> @@ -723,8 +717,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>
> rcu_read_lock();
>
> - for_each_tdp_pte_min_level(iter, root->spt, root->role.level,
> - min_level, start, end) {
> + tdp_root_for_each_pte(iter, root, start, end) {
> retry:
> if (can_yield &&
> tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) {
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>
