linux-kernel - [PATCH v2 2/8] x86/mm: Don't do a TLB flush if changing a PTE that isn't marked present

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Message-Id: <20231121212016.1154303-3-mhklinux@outlook.com>
Date:   Tue, 21 Nov 2023 13:20:10 -0800
From:   mhkelley58@...il.com
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        kirill.shutemov@...ux.intel.com, kys@...rosoft.com,
        haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
        luto@...nel.org, peterz@...radead.org, akpm@...ux-foundation.org,
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        thomas.lendacky@....com, ardb@...nel.org, jroedel@...e.de,
        seanjc@...gle.com, rick.p.edgecombe@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-hyperv@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v2 2/8] x86/mm: Don't do a TLB flush if changing a PTE that isn't marked present

From: Michael Kelley <mhklinux@...look.com>

The core function __change_page_attr() currently sets up a TLB flush if
a PTE is changed. But if the old value of the PTE doesn't include the
PRESENT flag, the PTE won't be in the TLB, so a flush isn't needed.

Avoid an unnecessary TLB flush by conditioning the flush on the old
PTE value including PRESENT.  This change improves the performance of
functions like set_memory_p() by avoiding the flush if the memory range
was previously all not present.

Signed-off-by: Michael Kelley <mhklinux@...look.com>
---
 arch/x86/mm/pat/set_memory.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 8e19796e7ce5..d7ef8d312a47 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -1636,7 +1636,10 @@ static int __change_page_attr(struct cpa_data *cpa, int primary)
 		 */
 		if (pte_val(old_pte) != pte_val(new_pte)) {
 			set_pte_atomic(kpte, new_pte);
-			cpa->flags |= CPA_FLUSHTLB;
+
+			/* If old_pte isn't present, it's not in the TLB */
+			if (pte_present(old_pte))
+				cpa->flags |= CPA_FLUSHTLB;
 		}
 		cpa->numpages = 1;
 		return 0;
-- 
2.25.1