linux-kernel - [PATCH v3 2/2] x86/mm/ident_map: Use gbpages only where full GB page should be mapped.

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Message-Id: <20240717213121.3064030-3-steve.wahl@hpe.com>
Date: Wed, 17 Jul 2024 16:31:21 -0500
From: Steve Wahl <steve.wahl@....com>
To: Steve Wahl <steve.wahl@....com>, Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Pavin Joseph <me@...injoseph.com>, Eric Hagberg <ehagberg@...il.com>
Cc: Simon Horman <horms@...ge.net.au>, Eric Biederman <ebiederm@...ssion.com>,
        Dave Young <dyoung@...hat.com>, Sarah Brofeldt <srhb@....dk>,
        Russ Anderson <rja@....com>, Dimitri Sivanich <sivanich@....com>,
        Hou Wenlong <houwenlong.hwl@...group.com>,
        Andrew Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>,
        Yuntao Wang <ytcoode@...il.com>, Bjorn Helgaas <bhelgaas@...gle.com>,
        Joerg Roedel <jroedel@...e.de>, Michael Roth <michael.roth@....com>,
        Tao Liu <ltao@...hat.com>, kexec@...ts.infradead.org,
        "Kalra, Ashish" <ashish.kalra@....com>,
        Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org
Subject: [PATCH v3 2/2] x86/mm/ident_map: Use gbpages only where full GB page should be mapped.

When ident_pud_init() uses only gbpages to create identity maps, large
ranges of addresses not actually requested can be included in the
resulting table; a 4K request will map a full GB.  This can include a
lot of extra address space past that requested, including areas marked
reserved by the BIOS.  That allows processor speculation into reserved
regions, that on UV systems can cause system halts.

Only use gbpages when map creation requests include the full GB page
of space.  Fall back to using smaller 2M pages when only portions of a
GB page are included in the request.

No attempt is made to coalesce mapping requests. If a request requires
a map entry at the 2M (pmd) level, subsequent mapping requests within
the same 1G region will also be at the pmd level, even if adjacent or
overlapping such requests could have been combined to map a full
gbpage.  Existing usage starts with larger regions and then adds
smaller regions, so this should not have any great consequence.

Signed-off-by: Steve Wahl <steve.wahl@....com>
Tested-by: Pavin Joseph <me@...injoseph.com>
Tested-by: Sarah Brofeldt <srhb@....dk>
Tested-by: Eric Hagberg <ehagberg@...il.com>
---
 arch/x86/mm/ident_map.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/ident_map.c b/arch/x86/mm/ident_map.c
index 968d7005f4a7..a204a332c71f 100644
--- a/arch/x86/mm/ident_map.c
+++ b/arch/x86/mm/ident_map.c
@@ -26,18 +26,31 @@ static int ident_pud_init(struct x86_mapping_info *info, pud_t *pud_page,
 	for (; addr < end; addr = next) {
 		pud_t *pud = pud_page + pud_index(addr);
 		pmd_t *pmd;
+		bool use_gbpage;
 
 		next = (addr & PUD_MASK) + PUD_SIZE;
 		if (next > end)
 			next = end;
 
-		if (info->direct_gbpages) {
-			pud_t pudval;
+		/* if this is already a gbpage, this portion is already mapped */
+		if (pud_leaf(*pud))
+			continue;
+
+		/* Is using a gbpage allowed? */
+		use_gbpage = info->direct_gbpages;
 
-			if (pud_present(*pud))
-				continue;
+		/* Don't use gbpage if it maps more than the requested region. */
+		/* at the begining: */
+		use_gbpage &= ((addr & ~PUD_MASK) == 0);
+		/* ... or at the end: */
+		use_gbpage &= ((next & ~PUD_MASK) == 0);
+
+		/* Never overwrite existing mappings */
+		use_gbpage &= !pud_present(*pud);
+
+		if (use_gbpage) {
+			pud_t pudval;
 
-			addr &= PUD_MASK;
 			pudval = __pud((addr - info->offset) | info->page_flag);
 			set_pud(pud, pudval);
 			continue;
-- 
2.26.2