Re: [RFC] [PATCH] cache pollution aware __copy_from_user_ll()

Hiro Yoshioka
2005-08-18T09:45:56Z

Chuck,

On 8/18/05, Chuck Ebbert <76306.1226@compuserve.com> wrote:
> On Wed, 17 Aug 2005 at 13:50:22 +0900 (JST), Hiro Yoshioka wrote:
>
> > 3) page faults/exceptions/...
> > 3-1 TS flag is set by the CPU (Am I right?)
>
> TS will _not_ be set if a trap/fault or interrupt occurs. The only
> way that could happen automatically would be to use a separate hardware
> task with its own TSS to handle those.

OK.

> And since the kernel does not have any state information of its own
> (no task_struct) any attempt to save the kernel-mode FPU state would
> overwrite the current user-mode state anyway.
>
> Interrupt and fault handlers will not use FP instructions anyway.
> The only thing you have to worry about is getting scheduled away
> while your code is running, and I guess that's why you have to worry
> about page faults. And as Arjan pointed out, if you are doing
> __copy_from_user_inatomic you cannot sleep (==switch to another task.)
>
> So I would try the code from include/asm-i386/xor.h, modify it to
> save as many registers as you plan to use and see what happens. It will
> do all the right things. See the xor_sse_2() for how to save and restore
> properly -- you will need to put your xmm_save area on the stack.

My hack is the following. I just change from using kernel_fpu_begin()
and kernel_fpu_end() to using a stack.

My test does not find any regressions.

--- usercopy.c.orig 2005-08-05 16:04:37.000000000 +0900
+++ usercopy.c 2005-08-18 16:53:37.000000000 +0900
@@ -10,6 +10,7 @@
#include <linux/highmem.h>
#include <linux/blkdev.h>
#include <linux/module.h>
+#include <asm/i387.h>
#include <asm/uaccess.h>
#include <asm/mmx.h>

@@ -511,6 +512,144 @@
: "memory"); \
} while (0)

+#define MMX_SAVE do { \
+ preempt_disable(); \
+ __asm__ __volatile__ ( \
+ "movl %%cr0,%0 ;\n\t" \
+ "clts ;\n\t" \
+ "movq %%mm0,(%1) ;\n\t" \
+ "movq %%mm1,8(%1) ;\n\t" \
+ "movq %%mm2,16(%1) ;\n\t" \
+ "movq %%mm3,24(%1) ;\n\t" \
+ : "=&r" (cr0) \
+ : "r" (mmx_save) \
+ : "memory"); \
+} while(0)
+
+#define MMX_RESTORE do { \
+ __asm__ __volatile__ ( \
+ "sfence ;\n\t" \
+ "movq (%1),%%mm0 ;\n\t" \
+ "movq 8(%1),%%mm1 ;\n\t" \
+ "movq 16(%1),%%mm2 ;\n\t" \
+ "movq 24(%1),%%mm3 ;\n\t" \
+ "movl %0,%%cr0 ;\n\t" \
+ : \
+ : "r" (cr0), "r" (mmx_save) \
+ : "memory"); \
+ preempt_enable(); \
+} while(0)
+
+#define ALIGN8 __attribute__((aligned(8)))
+
+/* Non Temporal Hint version of mmx_memcpy */
+/* It is cache aware */
+/* hyoshiok@miraclelinux.com */
+static unsigned long
+__copy_user_zeroing_nocache(void *to, const void *from, size_t len)
+{
+ /* Note! gcc doesn't seem to align stack variables properly, so we
+ * need to make use of unaligned loads and stores.
+ */
+ void *p;
+ int i;
+ char mmx_save[8*4] ALIGN8;
+ int cr0;
+
+ if (unlikely(in_interrupt())){
+ __copy_user_zeroing(to, from, len);
+ return len;
+ }
+
+ p = to;
+ i = len >> 6; /* len/64 */
+
+ /* kernel_fpu_begin();*/
+ MMX_SAVE;
+
+ __asm__ __volatile__ (
+ "1: prefetchnta (%0)\n" /* This set is 28 bytes */
+ " prefetchnta 64(%0)\n"
+ " prefetchnta 128(%0)\n"
+ " prefetchnta 192(%0)\n"
+ " prefetchnta 256(%0)\n"
+ "2: \n"
+ ".section .fixup, \"ax\"\n"
+ "3: movw $0x1AEB, 1b\n" /* jmp on 26 bytes */
+ " jmp 2b\n"
+ ".previous\n"
+ ".section __ex_table,\"a\"\n"
+ " .align 4\n"
+ " .long 1b, 3b\n"
+ ".previous"
+ : : "r" (from) );
+
+ for(; i>5; i--)
+ {
+ __asm__ __volatile__ (
+ "1: prefetchnta 320(%0)\n"
+ "2: movq (%0), %%mm0\n"
+ " movq 8(%0), %%mm1\n"
+ " movq 16(%0), %%mm2\n"
+ " movq 24(%0), %%mm3\n"
+ " movntq %%mm0, (%1)\n"
+ " movntq %%mm1, 8(%1)\n"
+ " movntq %%mm2, 16(%1)\n"
+ " movntq %%mm3, 24(%1)\n"
+ " movq 32(%0), %%mm0\n"
+ " movq 40(%0), %%mm1\n"
+ " movq 48(%0), %%mm2\n"
+ " movq 56(%0), %%mm3\n"
+ " movntq %%mm0, 32(%1)\n"
+ " movntq %%mm1, 40(%1)\n"
+ " movntq %%mm2, 48(%1)\n"
+ " movntq %%mm3, 56(%1)\n"
+ ".section .fixup, \"ax\"\n"
+ "3: movw $0x05EB, 1b\n" /* jmp on 5 bytes */
+ " jmp 2b\n"
+ ".previous\n"
+ ".section __ex_table,\"a\"\n"
+ " .align 4\n"
+ " .long 1b, 3b\n"
+ ".previous"
+ : : "r" (from), "r" (to) : "memory");
+ from+=64;
+ to+=64;
+ }
+
+ for(; i>0; i--)
+ {
+ __asm__ __volatile__ (
+ " movq (%0), %%mm0\n"
+ " movq 8(%0), %%mm1\n"
+ " movq 16(%0), %%mm2\n"
+ " movq 24(%0), %%mm3\n"
+ " movntq %%mm0, (%1)\n"
+ " movntq %%mm1, 8(%1)\n"
+ " movntq %%mm2, 16(%1)\n"
+ " movntq %%mm3, 24(%1)\n"
+ " movq 32(%0), %%mm0\n"
+ " movq 40(%0), %%mm1\n"
+ " movq 48(%0), %%mm2\n"
+ " movq 56(%0), %%mm3\n"
+ " movntq %%mm0, 32(%1)\n"
+ " movntq %%mm1, 40(%1)\n"
+ " movntq %%mm2, 48(%1)\n"
+ " movntq %%mm3, 56(%1)\n"
+ : : "r" (from), "r" (to) : "memory");
+ from+=64;
+ to+=64;
+ }
+ /*
+ * Now do the tail of the block
+ */
+ /* kernel_fpu_end();*/
+ MMX_RESTORE;
+ if(i=(len&63))
+ __copy_user_zeroing(to, from, i);
+ return i;
+}
+

unsigned long __copy_to_user_ll(void __user *to, const void *from,
unsigned long n)
{
@@ -582,6 +721,21 @@
return n;
}

+unsigned long
+__copy_from_user_ll_nocache(void *to, const void __user *from, unsigned long n)
+{
+ BUG_ON((long)n < 0);
+ if (n < 512) {
+ if (movsl_is_ok(to, from, n))
+ __copy_user_zeroing(to, from, n);
+ else
+ n = __copy_user_zeroing_intel(to, from, n);
+ }
+ else
+ n = __copy_user_zeroing_nocache(to, from, n);
+ return n;
+}
+
/**
* copy_to_user: - Copy a block of data into user space.
* @to: Destination address, in user space.

--
Hiro Yoshioka
mailto:hyoshiok at miraclelinux.com