x86: MMIO and gcc re-ordering issue

On Tue, 27 May 2008, Linus Torvalds wrote:
>
> Expecting people to fix up all drivers is simply not going to happen. And
> serializing things shouldn't be *that* expensive. People who cannot take
> the expense can continue to use the magic __raw_writel() etc stuff.

Of course, for non-x86, you kind of have to expect drivers to be
well-behaved, so non-x86 can probably avoid this simply because there are
less relevant drivers involved.

Here's a UNTESTED patch for x86 that may or may not compile and work, and
which serializes (on a compiler level) the IO accesses against regular
memory accesses.

__read[bwlq]()/__write[bwlq]() are not serialized with a :"memory"
barrier, although since they still use "asm volatile" I suspect that i
practice they are probably serial too. Did not look very closely at any
generated code (only did a trivial test to see that the code looks
*roughly* correct).

Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/io.h

@@ -3,6 +3,62 @@
 
 #define ARCH_HAS_IOREMAP_WC
 
+#include <linux/compiler.h>
+
+#define build_mmio_read(name, size, type, reg, barrier) \
+static inline type name(const volatile void __iomem *addr) \
+{ type ret; asm volatile("mov" size " %1,%0":"=" reg (ret) \
+:"m" (*(volatile type __force *)addr) barrier); return ret; }
+
+#define build_mmio_write(name, size, type, reg, barrier) \
+static inline void name(type val, volatile void __iomem *addr) \
+{ asm volatile("mov" size " %0,%1": :reg (val), \
+"m" (*(volatile type __force *)addr) barrier); }
+
+build_mmio_read(readb, "b", unsigned char, "q", :"memory")
+build_mmio_read(readw, "w", unsigned short, "r", :"memory")
+build_mmio_read(readl, "l", unsigned int, "r", :"memory")
+
+build_mmio_read(__readb, "b", unsigned char, "q", )
+build_mmio_read(__readw, "w", unsigned short, "r", )
+build_mmio_read(__readl, "l", unsigned int, "r", )
+
+build_mmio_write(writeb, "b", unsigned char, "q", :"memory")
+build_mmio_write(writew, "w", unsigned short, "r", :"memory")
+build_mmio_write(writel, "l", unsigned int, "r", :"memory")
+
+build_mmio_write(__writeb, "b", unsigned char, "q", )
+build_mmio_write(__writew, "w", unsigned short, "r", )
+build_mmio_write(__writel, "l", unsigned int, "r", )
+
+#define readb_relaxed(a) __readb(a)
+#define readw_relaxed(a) __readw(a)
+#define readl_relaxed(a) __readl(a)
+#define __raw_readb __readb
+#define __raw_readw __readw
+#define __raw_readl __readl
+
+#define __raw_writeb __writeb
+#define __raw_writew __writew
+#define __raw_writel __writel
+
+#define mmiowb() barrier()
+
+#ifdef CONFIG_X86_64
+build_mmio_read(readq, "q", unsigned long, "r", :"memory")
+build_mmio_read(__readq, "q", unsigned long, "r", )
+build_mmio_write(writeq, "q", unsigned long, "r", :"memory")
+build_mmio_write(__writeq, "q", unsigned long, "r", )
+
+#define readq_relaxed(a) __readq(a)
+#define __raw_readq __readq
+#define __raw_writeq writeq
+
+/* Let people know we have them */
+#define readq readq
+#define writeq writeq
+#endif
+
 #ifdef CONFIG_X86_32
 # include "io_32.h"
 #else