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From: Andy Lutomirski <luto@kernel.org>
To: x86@kernel.org
Cc: LKML <linux-kernel@vger.kernel.org>,
	"Krzysztof Mazur" <krzysiek@podlesie.net>,
	"Krzysztof Olędzki" <ole@ans.pl>, "Arnd Bergmann" <arnd@arndb.de>,
	"Andy Lutomirski" <luto@kernel.org>
Subject: [PATCH v2 2/4] x86/mmx: Use KFPU_387 for MMX string operations
Date: Tue, 19 Jan 2021 09:39:00 -0800	[thread overview]
Message-ID: <cd68d61ece035201804a06e6993a2bd06bfe298b.1611077835.git.luto@kernel.org> (raw)
In-Reply-To: <cover.1611077835.git.luto@kernel.org>

The default kernel_fpu_begin() doesn't work on systems that support XMM but
haven't yet enabled CR4.OSFXSR.  This causes crashes when _mmx_memcpy() is
called too early because LDMXCSR generates #UD when the aforementioned bit
is clear.

Fix it by using kernel_fpu_begin_mask(KFPU_387) explicitly.

Fixes: 7ad816762f9b ("x86/fpu: Reset MXCSR to default in kernel_fpu_begin()")
Reported-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/lib/mmx_32.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/lib/mmx_32.c b/arch/x86/lib/mmx_32.c
index 4321fa02e18d..2a6ad7aa148a 100644
--- a/arch/x86/lib/mmx_32.c
+++ b/arch/x86/lib/mmx_32.c
@@ -26,6 +26,16 @@
 #include <asm/fpu/api.h>
 #include <asm/asm.h>
 
+/*
+ * For MMX, we use KFPU_387.  MMX instructions are not affected by MXCSR,
+ * but both AMD and Intel documentation states that even integer MMX
+ * operations will result in #MF if an exception is pending in FCW.
+ *
+ * We don't need EMMS afterwards because, after we call kernel_fpu_end(),
+ * any subsequent user of the 387 stack will reinitialize it using
+ * KFPU_387.
+ */
+
 void *_mmx_memcpy(void *to, const void *from, size_t len)
 {
 	void *p;
@@ -37,7 +47,7 @@ void *_mmx_memcpy(void *to, const void *from, size_t len)
 	p = to;
 	i = len >> 6; /* len/64 */
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"1: prefetch (%0)\n"		/* This set is 28 bytes */
@@ -127,7 +137,7 @@ static void fast_clear_page(void *page)
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"  pxor %%mm0, %%mm0\n" : :
@@ -160,7 +170,7 @@ static void fast_copy_page(void *to, void *from)
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	/*
 	 * maybe the prefetch stuff can go before the expensive fnsave...
@@ -247,7 +257,7 @@ static void fast_clear_page(void *page)
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"  pxor %%mm0, %%mm0\n" : :
@@ -282,7 +292,7 @@ static void fast_copy_page(void *to, void *from)
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"1: prefetch (%0)\n"
-- 
2.29.2


  parent reply	other threads:[~2021-01-19 18:56 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-19 17:38 [PATCH v2 0/4] x86/fpu: Reduce unnecessary FNINIT and MXCSR usage Andy Lutomirski
2021-01-19 17:38 ` [PATCH v2 1/4] x86/fpu: Add kernel_fpu_begin_mask() to selectively initialize state Andy Lutomirski
2021-01-19 19:34   ` Sean Christopherson
2021-01-20 11:53   ` Borislav Petkov
2021-01-20 12:19     ` Andy Lutomirski
2021-01-19 17:39 ` Andy Lutomirski [this message]
2021-01-20 18:18   ` [PATCH v2 2/4] x86/mmx: Use KFPU_387 for MMX string operations Borislav Petkov
2021-01-19 17:39 ` [PATCH v2 3/4] x86/fpu: Make the EFI FPU calling convention explicit Andy Lutomirski
2021-01-19 17:39 ` [PATCH v2 4/4] x86/fpu/64: Don't FNINIT in kernel_fpu_begin() Andy Lutomirski
2021-01-20 10:07   ` Peter Zijlstra
2021-01-20 18:28     ` Borislav Petkov
2021-01-20  7:51 ` [PATCH v2 0/4] x86/fpu: Reduce unnecessary FNINIT and MXCSR usage Krzysztof Olędzki
2021-01-21  5:03   ` Andy Lutomirski

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