Re: [PATCH v2 01/10] KVM: Assert that notifier count is elevated in .change_pte()

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/04/21 02:56, Sean Christopherson wrote:
> In KVM's .change_pte() notification callback, replace the notifier
> sequence bump with a WARN_ON assertion that the notifier count is
> elevated.  An elevated count provides stricter protections than bumping
> the sequence, and the sequence is guarnateed to be bumped before the
> count hits zero.
> 
> When .change_pte() was added by commit 828502d30073 ("ksm: add
> mmu_notifier set_pte_at_notify()"), bumping the sequence was necessary
> as .change_pte() would be invoked without any surrounding notifications.
> 
> However, since commit 6bdb913f0a70 ("mm: wrap calls to set_pte_at_notify
> with invalidate_range_start and invalidate_range_end"), all calls to
> .change_pte() are guaranteed to be bookended by start() and end(), and
> so are guaranteed to run with an elevated notifier count.
> 
> Note, wrapping .change_pte() with .invalidate_range_{start,end}() is a
> bug of sorts, as invalidating the secondary MMU's (KVM's) PTE defeats
> the purpose of .change_pte().  Every arch's kvm_set_spte_hva() assumes
> .change_pte() is called when the relevant SPTE is present in KVM's MMU,
> as the original goal was to accelerate Kernel Samepage Merging (KSM) by
> updating KVM's SPTEs without requiring a VM-Exit (due to invalidating
> the SPTE).  I.e. it means that .change_pte() is effectively dead code
> on _all_ architectures.
> 
> x86 and MIPS are clearcut nops if the old SPTE is not-present, and that
> is guaranteed due to the prior invalidation.  PPC simply unmaps the SPTE,
> which again should be a nop due to the invalidation.  arm64 is a bit
> murky, but it's also likely a nop because kvm_pgtable_stage2_map() is
> called without a cache pointer, which means it will map an entry if and
> only if an existing PTE was found.
> 
> For now, take advantage of the bug to simplify future consolidation of
> KVMs's MMU notifier code.   Doing so will not greatly complicate fixing
> .change_pte(), assuming it's even worth fixing.  .change_pte() has been
> broken for 8+ years and no one has complained.  Even if there are
> KSM+KVM users that care deeply about its performance, the benefits of
> avoiding VM-Exits via .change_pte() need to be reevaluated to justify
> the added complexity and testing burden.  Ripping out .change_pte()
> entirely would be a lot easier.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   virt/kvm/kvm_main.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d1de843b7618..8df091950161 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -461,12 +461,17 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
>   
>   	trace_kvm_set_spte_hva(address);
>   
> +	/*
> +	 * .change_pte() must be bookended by .invalidate_range_{start,end}(),

Changed to "surrounded" for the benefit of non-native speakers. :)

Paolo

> +	 * and so always runs with an elevated notifier count.  This obviates
> +	 * the need to bump the sequence count.
> +	 */
> +	WARN_ON_ONCE(!kvm->mmu_notifier_count);
> +
>   	idx = srcu_read_lock(&kvm->srcu);
>   
>   	KVM_MMU_LOCK(kvm);
>   
> -	kvm->mmu_notifier_seq++;
> -
>   	if (kvm_set_spte_hva(kvm, address, pte))
>   		kvm_flush_remote_tlbs(kvm);
>   
> 

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