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

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

[Sean Christopherson]

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)

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

[Kai Huang]

On Tue, 2022-07-12 at 17:47 +0000, Sean Christopherson wrote:
> 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)

Yes looks better.  Will send out  a new patch soon.  Thanks.

-- 
Thanks,
-Kai