Re: [PATCH] KVM, x86/mmu: Fix the comment around kvm_tdp_mmu_zap_leafs()

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 12, 2022, Kai Huang wrote:
> Now kvm_tdp_mmu_zap_leafs() only zaps leaf SPTEs but not any non-root
> pages within that GFN range anymore, so the comment isn't right.
> 
> Fixes: f47e5bbbc92f ("KVM: x86/mmu: Zap only TDP MMU leafs in zap range and mmu_notifier unmap")
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index f3a430d64975..7692e6273462 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -969,10 +969,9 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
>  }
>  
>  /*
> - * Tears down the mappings for the range of gfns, [start, end), and frees the
> - * non-root pages mapping GFNs strictly within that range. Returns true if
> - * SPTEs have been cleared and a TLB flush is needed before releasing the
> - * MMU lock.
> + * Zap leafs SPTEs for the range of gfns, [start, end) for all roots. Returns
> + * true if SPTEs have been cleared and a TLB flush is needed before releasing
> + * the MMU lock.

What about shifting the comment from tdp_mmu_zap_leafs() instead of duplicating it?
tdp_mmu_zap_leafs() is static and kvm_tdp_mmu_zap_leafs() is the sole caller.  And
opportunistically tweak the blurb about SPTEs being cleared to (a) say "zapped"
instead of "cleared" because "cleared" will be wrong if/when KVM sets SUPPRESS_VE,
and (b) to clarify that a flush is needed if and only if a SPTE has been zapped
since MMU lock was last acquired.

E.g.

/*
 * If can_yield is true, will release the MMU lock and reschedule if the
 * scheduler needs the CPU or there is contention on the MMU lock. If this
 * function cannot yield, it will not release the MMU lock or reschedule and
 * the caller must ensure it does not supply too large a GFN range, or the
 * operation can cause a soft lockup.
 */
static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
			      gfn_t start, gfn_t end, bool can_yield, bool flush)

/*
 * Zap leafs SPTEs for the range of gfns, [start, end), for all roots.  Returns
 * true if a TLB flush is needed before releasing the MMU lock, i.e. if one or
 * more SPTEs were zapped since the MMU lock was last acquired.
 */
bool kvm_tdp_mmu_zap_leafs(struct kvm *kvm, int as_id, gfn_t start, gfn_t end,
			   bool can_yield, bool flush)