linux-kernel - [tip: x86/urgent] x86/PAT: Have pat_enabled() properly reflect state when running on Xen

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Message-ID: <166055884287.401.612271624942869534.tip-bot2@tip-bot2>
Date:   Mon, 15 Aug 2022 10:20:42 -0000
From:   "tip-bot2 for Jan Beulich" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Jan Beulich <jbeulich@...e.com>, Borislav Petkov <bp@...e.de>,
        Ingo Molnar <mingo@...nel.org>, <stable@...r.kernel.org>,
        Juergen Gross <jgross@...e.com>,
        Lucas De Marchi <lucas.demarchi@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/PAT: Have pat_enabled() properly reflect state
 when running on Xen

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     72cbc8f04fe2fa93443c0fcccb7ad91dfea3d9ce
Gitweb:        https://git.kernel.org/tip/72cbc8f04fe2fa93443c0fcccb7ad91dfea3d9ce
Author:        Jan Beulich <jbeulich@...e.com>
AuthorDate:    Thu, 28 Apr 2022 16:50:29 +02:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Mon, 15 Aug 2022 10:51:23 +02:00

x86/PAT: Have pat_enabled() properly reflect state when running on Xen

After commit ID in the Fixes: tag, pat_enabled() returns false (because
of PAT initialization being suppressed in the absence of MTRRs being
announced to be available).

This has become a problem: the i915 driver now fails to initialize when
running PV on Xen (i915_gem_object_pin_map() is where I located the
induced failure), and its error handling is flaky enough to (at least
sometimes) result in a hung system.

Yet even beyond that problem the keying of the use of WC mappings to
pat_enabled() (see arch_can_pci_mmap_wc()) means that in particular
graphics frame buffer accesses would have been quite a bit less optimal
than possible.

Arrange for the function to return true in such environments, without
undermining the rest of PAT MSR management logic considering PAT to be
disabled: specifically, no writes to the PAT MSR should occur.

For the new boolean to live in .init.data, init_cache_modes() also needs
moving to .init.text (where it could/should have lived already before).

  [ bp: This is the "small fix" variant for stable. It'll get replaced
    with a proper PAT and MTRR detection split upstream but that is too
    involved for a stable backport.
    - additional touchups to commit msg. Use cpu_feature_enabled(). ]

Fixes: bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT with pat_enabled()")
Signed-off-by: Jan Beulich <jbeulich@...e.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Acked-by: Ingo Molnar <mingo@...nel.org>
Cc: <stable@...r.kernel.org>
Cc: Juergen Gross <jgross@...e.com>
Cc: Lucas De Marchi <lucas.demarchi@...el.com>
Link: https://lore.kernel.org/r/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com
---
 arch/x86/mm/pat/memtype.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index d5ef64d..66a209f 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -62,6 +62,7 @@
 
 static bool __read_mostly pat_bp_initialized;
 static bool __read_mostly pat_disabled = !IS_ENABLED(CONFIG_X86_PAT);
+static bool __initdata pat_force_disabled = !IS_ENABLED(CONFIG_X86_PAT);
 static bool __read_mostly pat_bp_enabled;
 static bool __read_mostly pat_cm_initialized;
 
@@ -86,6 +87,7 @@ void pat_disable(const char *msg_reason)
 static int __init nopat(char *str)
 {
 	pat_disable("PAT support disabled via boot option.");
+	pat_force_disabled = true;
 	return 0;
 }
 early_param("nopat", nopat);
@@ -272,7 +274,7 @@ static void pat_ap_init(u64 pat)
 	wrmsrl(MSR_IA32_CR_PAT, pat);
 }
 
-void init_cache_modes(void)
+void __init init_cache_modes(void)
 {
 	u64 pat = 0;
 
@@ -313,6 +315,12 @@ void init_cache_modes(void)
 		 */
 		pat = PAT(0, WB) | PAT(1, WT) | PAT(2, UC_MINUS) | PAT(3, UC) |
 		      PAT(4, WB) | PAT(5, WT) | PAT(6, UC_MINUS) | PAT(7, UC);
+	} else if (!pat_force_disabled && cpu_feature_enabled(X86_FEATURE_HYPERVISOR)) {
+		/*
+		 * Clearly PAT is enabled underneath. Allow pat_enabled() to
+		 * reflect this.
+		 */
+		pat_bp_enabled = true;
 	}
 
 	__init_cache_modes(pat);