Re: [PATCH v2 25/30] KVM: x86/mmu: Zap roots in two passes to avoid inducing RCU stalls

[David Matlack]

On Thu, Dec 23, 2021 at 10:23:13PM +0000, Sean Christopherson wrote:
> When zapping a TDP MMU root, perform the zap in two passes to avoid
> zapping an entire top-level SPTE while holding RCU, which can induce RCU
> stalls.  In the first pass, zap SPTEs at PG_LEVEL_1G, and then
> zap top-level entries in the second pass.
> 
> With 4-level paging, zapping a PGD that is fully populated with 4kb leaf
> SPTEs take up to ~7 or so seconds (time varies based number of kernel
> config, CPUs, vCPUs, etc...).  With 5-level paging, that time can balloon
> well into hundreds of seconds.
> 
> Before remote TLB flushes were omitted, the problem was even worse as
> waiting for all active vCPUs to respond to the IPI introduced significant
> overhead for VMs with large numbers of vCPUs.
> 
> By zapping 1gb SPTEs (both shadow pages and hugepages) in the first pass,
> the amount of work that is done without dropping RCU protection is
> strictly bounded, with the worst case latency for a single operation
> being less than 100ms.
> 
> Zapping at 1gb in the first pass is not arbitrary.  First and foremost,
> KVM relies on being able to zap 1gb shadow pages in a single shot when
> when repacing a shadow page with a hugepage.

When dirty logging is disabled, zap_collapsible_spte_range() does the
bulk of the work zapping leaf SPTEs and allows yielding. I guess that
could race with a vCPU faulting in the huge page though and the vCPU
could do the bulk of the work.

Are there any other scenarios where KVM relies on zapping 1GB worth of
4KB SPTEs without yielding?

In any case, 100ms is a long time to hog the CPU. Why not just do the
safe thing and zap each level? 4K, then 2M, then 1GB, ..., then root
level. The only argument against it I can think of is performance (lots
of redundant walks through the page table). But I don't think root
zapping is especially latency critical.

> Zapping a 1gb shadow page
> that is fully populated with 4kb dirty SPTEs also triggers the worst case
> latency due writing back the struct page accessed/dirty bits for each 4kb
> page, i.e. the two-pass approach is guaranteed to work so long as KVM can
> cleany zap a 1gb shadow page.
> 
>   rcu: INFO: rcu_sched self-detected stall on CPU
>   rcu:     52-....: (20999 ticks this GP) idle=7be/1/0x4000000000000000
>                                           softirq=15759/15759 fqs=5058
>    (t=21016 jiffies g=66453 q=238577)
>   NMI backtrace for cpu 52
>   Call Trace:
>    ...
>    mark_page_accessed+0x266/0x2f0
>    kvm_set_pfn_accessed+0x31/0x40
>    handle_removed_tdp_mmu_page+0x259/0x2e0
>    __handle_changed_spte+0x223/0x2c0
>    handle_removed_tdp_mmu_page+0x1c1/0x2e0
>    __handle_changed_spte+0x223/0x2c0
>    handle_removed_tdp_mmu_page+0x1c1/0x2e0
>    __handle_changed_spte+0x223/0x2c0
>    zap_gfn_range+0x141/0x3b0
>    kvm_tdp_mmu_zap_invalidated_roots+0xc8/0x130
>    kvm_mmu_zap_all_fast+0x121/0x190
>    kvm_mmu_invalidate_zap_pages_in_memslot+0xe/0x10
>    kvm_page_track_flush_slot+0x5c/0x80
>    kvm_arch_flush_shadow_memslot+0xe/0x10
>    kvm_set_memslot+0x172/0x4e0
>    __kvm_set_memory_region+0x337/0x590
>    kvm_vm_ioctl+0x49c/0xf80
> 
> Reported-by: David Matlack <dmatlack@xxxxxxxxxx>
> Cc: Ben Gardon <bgardon@xxxxxxxxxx>
> Cc: Mingwei Zhang <mizhang@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index aec97e037a8d..2e28f5e4b761 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -809,6 +809,18 @@ static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
>  	gfn_t end = tdp_mmu_max_gfn_host();
>  	gfn_t start = 0;
>  
> +	/*
> +	 * To avoid RCU stalls due to recursively removing huge swaths of SPs,
> +	 * split the zap into two passes.  On the first pass, zap at the 1gb
> +	 * level, and then zap top-level SPs on the second pass.  "1gb" is not
> +	 * arbitrary, as KVM must be able to zap a 1gb shadow page without
> +	 * inducing a stall to allow in-place replacement with a 1gb hugepage.
> +	 *
> +	 * Because zapping a SP recurses on its children, stepping down to
> +	 * PG_LEVEL_4K in the iterator itself is unnecessary.
> +	 */
> +	int zap_level = PG_LEVEL_1G;
> +
>  	/*
>  	 * The root must have an elevated refcount so that it's reachable via
>  	 * mmu_notifier callbacks, which allows this path to yield and drop
> @@ -825,12 +837,9 @@ static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
>  
>  	rcu_read_lock();
>  
> -	/*
> -	 * No need to try to step down in the iterator when zapping an entire
> -	 * root, zapping an upper-level SPTE will recurse on its children.
> -	 */
> +start:
>  	for_each_tdp_pte_min_level(iter, root->spt, root->role.level,
> -				   root->role.level, start, end) {
> +				   zap_level, start, end) {
>  retry:
>  		if (tdp_mmu_iter_cond_resched(kvm, &iter, false, shared))
>  			continue;
> @@ -838,6 +847,9 @@ static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
>  		if (!is_shadow_present_pte(iter.old_spte))
>  			continue;
>  
> +		if (iter.level > zap_level)
> +			continue;
> +
>  		if (!shared) {
>  			tdp_mmu_set_spte(kvm, &iter, 0);
>  		} else if (!tdp_mmu_set_spte_atomic(kvm, &iter, 0)) {
> @@ -846,6 +858,11 @@ static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
>  		}
>  	}
>  
> +	if (zap_level < root->role.level) {
> +		zap_level = root->role.level;
> +		goto start;
> +	}

This is probably just person opinion but I find the 2 iteration goto
loop harder to understand than just open-coding the 2 passes.

e.g.

  static void tdp_mmu_zap_root(...)
  {
          /*
           * To avoid RCU stalls due to recursively removing huge swaths of SPs,
           * split the zap into two passes.  On the first pass, zap at the 1gb
           * level, and then zap top-level SPs on the second pass.  "1gb" is not
           * arbitrary, as KVM must be able to zap a 1gb shadow page without
           * inducing a stall to allow in-place replacement with a 1gb hugepage.
           *
           * Because zapping a SP recurses on its children, stepping down to
           * PG_LEVEL_4K in the iterator itself is unnecessary.
           */
          tdp_mmu_zap_root_level(..., PG_LEVEL_1G);
          tdp_mmu_zap_root_level(..., root->role.level);
  }

Or just go ahead and zap each level from 4K up to root->role.level as I
mentioned above.

> +
>  	rcu_read_unlock();
>  }
>  
> -- 
> 2.34.1.448.ga2b2bfdf31-goog
>