Re: [PATCH 06/24] kvm: x86/mmu: Skip no-op changes in TDP MMU functions
From: Ben Gardon <hidden>
Date: 2021-01-25 23:53:02
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On Wed, Jan 20, 2021 at 11:51 AM Sean Christopherson [off-list ref] wrote:
On Tue, Jan 12, 2021, Ben Gardon wrote:quoted
Skip setting SPTEs if no change is expected. Reviewed-by: Peter Feiner <redacted>Nit on all of these, can you remove the extra newline between the Reviewed-by and SOB?
Yeah, that line is annoying. I'll make sure it's not there on future patches.
quoted
Signed-off-by: Ben Gardon <redacted> --- 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.
I agree that' s cleaner. I'll group the checks in the next patch set version.
quoted hunk ↗ jump to hunk
quoted
+ 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.quoted
+ 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); } }
Great point, that doesn't work as intended at all. I'll adopt your proposed fix and include it in a patch after this one in the next version of the series.
quoted
new_spte = iter.old_spte | shadow_dirty_mask; tdp_mmu_set_spte(kvm, &iter, new_spte); -- 2.30.0.284.gd98b1dd5eaa7-goog