[PATCH v2] KVM, x86/mmu: Fix the comment around kvm_tdp_mmu_zap_leafs()

[PATCH v2] KVM, x86/mmu: Fix the comment around kvm_tdp_mmu_zap_leafs()

[Kai Huang]

Now kvm_tdp_mmu_zap_leafs() only zaps leaf SPTEs but not any non-root
pages within that GFN range anymore, so the comment around it isn't
right.

Fix it by shifting the comment from tdp_mmu_zap_leafs() instead of
duplicating it, as tdp_mmu_zap_leafs() is static and is only called by
kvm_tdp_mmu_zap_leafs().

Opportunistically tweak the blurb about SPTEs being cleared to (a) say
"zapped" instead of "cleared" because "cleared" will be wrong if/when
KVM allows a non-zero value for non-present SPTE (i.e. for Intel TDX),
and (b) to clarify that a flush is needed if and only if a SPTE has been
zapped since MMU lock was last acquired.

Fixes: f47e5bbbc92f ("KVM: x86/mmu: Zap only TDP MMU leafs in zap range and mmu_notifier unmap")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index f3a430d64975..58b34f7cd0f5 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -924,9 +924,6 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
 }
 
 /*
- * Zap leafs SPTEs for the range of gfns, [start, end). Returns true if SPTEs
- * have been cleared and a TLB flush is needed before releasing the MMU lock.
- *
  * 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
@@ -969,10 +966,9 @@ 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 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)
-- 
2.36.1

Re: [PATCH v2] KVM, x86/mmu: Fix the comment around kvm_tdp_mmu_zap_leafs()

[Sean Christopherson]

On Wed, Jul 13, 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 around it isn't
> right.
> 
> Fix it by shifting the comment from tdp_mmu_zap_leafs() instead of
> duplicating it, as tdp_mmu_zap_leafs() is static and is only called by
> kvm_tdp_mmu_zap_leafs().
> 
> Opportunistically tweak the blurb about SPTEs being cleared to (a) say
> "zapped" instead of "cleared" because "cleared" will be wrong if/when
> KVM allows a non-zero value for non-present SPTE (i.e. for Intel TDX),
> and (b) to clarify that a flush is needed if and only if a SPTE has been
> zapped since MMU lock was last acquired.
> 
> Fixes: f47e5bbbc92f ("KVM: x86/mmu: Zap only TDP MMU leafs in zap range and mmu_notifier unmap")
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index f3a430d64975..58b34f7cd0f5 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -924,9 +924,6 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
>  }
>  
>  /*
> - * Zap leafs SPTEs for the range of gfns, [start, end). Returns true if SPTEs
> - * have been cleared and a TLB flush is needed before releasing the MMU lock.
> - *
>   * 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
> @@ -969,10 +966,9 @@ 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

s/leafs/leaf (my fault, sorry)

With that tweak,

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH v2] KVM, x86/mmu: Fix the comment around kvm_tdp_mmu_zap_leafs()

[Kai Huang]

On Tue, 2022-07-19 at 19:02 +0000, Sean Christopherson wrote:
> On Wed, Jul 13, 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 around it isn't
> > right.
> > 
> > Fix it by shifting the comment from tdp_mmu_zap_leafs() instead of
> > duplicating it, as tdp_mmu_zap_leafs() is static and is only called by
> > kvm_tdp_mmu_zap_leafs().
> > 
> > Opportunistically tweak the blurb about SPTEs being cleared to (a) say
> > "zapped" instead of "cleared" because "cleared" will be wrong if/when
> > KVM allows a non-zero value for non-present SPTE (i.e. for Intel TDX),
> > and (b) to clarify that a flush is needed if and only if a SPTE has been
> > zapped since MMU lock was last acquired.
> > 
> > Fixes: f47e5bbbc92f ("KVM: x86/mmu: Zap only TDP MMU leafs in zap range and mmu_notifier unmap")
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Kai Huang <kai.huang@intel.com>
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.c | 10 +++-------
> >  1 file changed, 3 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index f3a430d64975..58b34f7cd0f5 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -924,9 +924,6 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> >  }
> >  
> >  /*
> > - * Zap leafs SPTEs for the range of gfns, [start, end). Returns true if SPTEs
> > - * have been cleared and a TLB flush is needed before releasing the MMU lock.
> > - *
> >   * 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
> > @@ -969,10 +966,9 @@ 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
> 
> s/leafs/leaf (my fault, sorry)
> 
> With that tweak,
> 
> Reviewed-by: Sean Christopherson <seanjc@google.com>

I should have checked error too :)

I have sent out v3 with above fixed.  Thanks.

-- 
Thanks,
-Kai