linux-kernel - Re: [PATCH 06/24] kvm: x86/mmu: Skip no-op changes in TDP MMU functions

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Date:   Wed, 20 Jan 2021 11:51:10 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 06/24] kvm: x86/mmu: Skip no-op changes in TDP MMU
 functions

On Tue, Jan 12, 2021, Ben Gardon wrote:
> Skip setting SPTEs if no change is expected.
> 
> Reviewed-by: Peter Feiner <pfeiner@...gle.com>
>
Nit on all of these, can you remove the extra newline between the Reviewed-by
and SOB?

> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 1987da0da66e..2650fa9fe066 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -882,6 +882,9 @@ static bool wrprot_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>  		    !is_last_spte(iter.old_spte, iter.level))
>  			continue;
>  
> +		if (!(iter.old_spte & PT_WRITABLE_MASK))

Include the new check with the existing if statement?  I think it makes sense to
group all the checks on old_spte.

> +			continue;
> +
>  		new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
>  
>  		tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
> @@ -1079,6 +1082,9 @@ static bool set_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>  		if (!is_shadow_present_pte(iter.old_spte))
>  			continue;
>  
> +		if (iter.old_spte & shadow_dirty_mask)

Same comment here.

> +			continue;
> +

Unrelated to this patch, but it got me looking at the code: shouldn't
clear_dirty_pt_masked() clear the bit in @mask before checking whether or not
the spte needs to be modified?  That way the early break kicks in after sptes
are checked, not necessarily written.  E.g.

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 2650fa9fe066..d8eeae910cbf 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1010,21 +1010,21 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
                    !(mask & (1UL << (iter.gfn - gfn))))
                        continue;

-               if (wrprot || spte_ad_need_write_protect(iter.old_spte)) {
-                       if (is_writable_pte(iter.old_spte))
-                               new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
-                       else
-                               continue;
-               } else {
-                       if (iter.old_spte & shadow_dirty_mask)
-                               new_spte = iter.old_spte & ~shadow_dirty_mask;
-                       else
-                               continue;
-               }
-
-               tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
-
                mask &= ~(1UL << (iter.gfn - gfn));
+
+               if (wrprot || spte_ad_need_write_protect(iter.old_spte)) {
+                       if (is_writable_pte(iter.old_spte))
+                               new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
+                       else
+                               continue;
+               } else {
+                       if (iter.old_spte & shadow_dirty_mask)
+                               new_spte = iter.old_spte & ~shadow_dirty_mask;
+                       else
+                               continue;
+               }
+
+               tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
        }
 }


>  		new_spte = iter.old_spte | shadow_dirty_mask;
>  
>  		tdp_mmu_set_spte(kvm, &iter, new_spte);
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
>