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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH RFC 05/25] Introduce clear_user
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Introduce clear_user for x86 and ia64, shamelessly taken from Linux.
The x86 version is the 32 bit clear_user implementation.
Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
---
xen/arch/ia64/linux/memcpy_mck.S | 177 ++++++++++++++++++++++++++++++++++++++
xen/arch/x86/usercopy.c | 36 ++++++++
xen/include/asm-ia64/uaccess.h | 12 +++
xen/include/asm-x86/uaccess.h | 1 +
4 files changed, 226 insertions(+), 0 deletions(-)
diff --git a/xen/arch/ia64/linux/memcpy_mck.S b/xen/arch/ia64/linux/memcpy_mck.S
index 6f308e6..8b07006 100644
--- a/xen/arch/ia64/linux/memcpy_mck.S
+++ b/xen/arch/ia64/linux/memcpy_mck.S
@@ -659,3 +659,180 @@ EK(.ex_handler, (p17) st8 [dst1]=r39,8);
\
/* end of McKinley specific optimization */
END(__copy_user)
+
+/*
+ * Theory of operations:
+ * - we check whether or not the buffer is small, i.e., less than 17
+ * in which case we do the byte by byte loop.
+ *
+ * - Otherwise we go progressively from 1 byte store to 8byte store in
+ * the head part, the body is a 16byte store loop and we finish we the
+ * tail for the last 15 bytes.
+ * The good point about this breakdown is that the long buffer handling
+ * contains only 2 branches.
+ *
+ * The reason for not using shifting & masking for both the head and the
+ * tail is to stay semantically correct. This routine is not supposed
+ * to write bytes outside of the buffer. While most of the time this would
+ * be ok, we can't tolerate a mistake. A classical example is the case
+ * of multithreaded code were to the extra bytes touched is actually owned
+ * by another thread which runs concurrently to ours. Another, less likely,
+ * example is with device drivers where reading an I/O mapped location may
+ * have side effects (same thing for writing).
+ */
+GLOBAL_ENTRY(__do_clear_user)
+ .prologue
+ .save ar.pfs, saved_pfs
+ alloc saved_pfs=ar.pfs,2,0,0,0
+ cmp.eq p6,p0=r0,len // check for zero length
+ .save ar.lc, saved_lc
+ mov saved_lc=ar.lc // preserve ar.lc (slow)
+ .body
+ ;; // avoid WAW on CFM
+ adds tmp=-1,len // br.ctop is repeat/until
+ mov ret0=len // return value is length at this point
+(p6) br.ret.spnt.many rp
+ ;;
+ cmp.lt p6,p0=16,len // if len > 16 then long memset
+ mov ar.lc=tmp // initialize lc for small count
+(p6) br.cond.dptk .long_do_clear
+ ;; // WAR on ar.lc
+ //
+ // worst case 16 iterations, avg 8 iterations
+ //
+ // We could have played with the predicates to use the extra
+ // M slot for 2 stores/iteration but the cost the initialization
+ // the various counters compared to how long the loop is supposed
+ // to last on average does not make this solution viable.
+ //
+1:
+ EX( .Lexit1, st1 [buf]=r0,1 )
+ adds len=-1,len // countdown length using len
+ br.cloop.dptk 1b
+ ;; // avoid RAW on ar.lc
+ //
+ // .Lexit4: comes from byte by byte loop
+ // len contains bytes left
+.Lexit1:
+ mov ret0=len // faster than using ar.lc
+ mov ar.lc=saved_lc
+ br.ret.sptk.many rp // end of short clear_user
+
+
+ //
+ // At this point we know we have more than 16 bytes to copy
+ // so we focus on alignment (no branches required)
+ //
+ // The use of len/len2 for countdown of the number of bytes left
+ // instead of ret0 is due to the fact that the exception code
+ // changes the values of r8.
+ //
+.long_do_clear:
+ tbit.nz p6,p0=buf,0 // odd alignment (for long_do_clear)
+ ;;
+ EX( .Lexit3, (p6) st1 [buf]=r0,1 ) // 1-byte aligned
+(p6) adds len=-1,len;; // sync because buf is modified
+ tbit.nz p6,p0=buf,1
+ ;;
+ EX( .Lexit3, (p6) st2 [buf]=r0,2 ) // 2-byte aligned
+(p6) adds len=-2,len;;
+ tbit.nz p6,p0=buf,2
+ ;;
+ EX( .Lexit3, (p6) st4 [buf]=r0,4 ) // 4-byte aligned
+(p6) adds len=-4,len;;
+ tbit.nz p6,p0=buf,3
+ ;;
+ EX( .Lexit3, (p6) st8 [buf]=r0,8 ) // 8-byte aligned
+(p6) adds len=-8,len;;
+ shr.u cnt=len,4 // number of 128-bit (2x64bit) words
+ ;;
+ cmp.eq p6,p0=r0,cnt
+ adds tmp=-1,cnt
+(p6) br.cond.dpnt .dotail // we have less than 16 bytes left
+ ;;
+ adds buf2=8,buf // setup second base pointer
+ mov ar.lc=tmp
+ ;;
+
+ //
+ // 16bytes/iteration core loop
+ //
+ // The second store can never generate a fault because
+ // we come into the loop only when we are 16-byte aligned.
+ // This means that if we cross a page then it will always be
+ // in the first store and never in the second.
+ //
+ //
+ // We need to keep track of the remaining length. A possible
(optimistic)
+ // way would be to use ar.lc and derive how many byte were left by
+ // doing : left= 16*ar.lc + 16. this would avoid the addition at
+ // every iteration.
+ // However we need to keep the synchronization point. A template
+ // M;;MB does not exist and thus we can keep the addition at no
+ // extra cycle cost (use a nop slot anyway). It also simplifies the
+ // (unlikely) error recovery code
+ //
+
+2: EX(.Lexit3, st8 [buf]=r0,16 )
+ ;; // needed to get len correct when error
+ st8 [buf2]=r0,16
+ adds len=-16,len
+ br.cloop.dptk 2b
+ ;;
+ mov ar.lc=saved_lc
+ //
+ // tail correction based on len only
+ //
+ // We alternate the use of len3,len2 to allow parallelism and correct
+ // error handling. We also reuse p6/p7 to return correct value.
+ // The addition of len2/len3 does not cost anything more compared to
+ // the regular memset as we had empty slots.
+ //
+.dotail:
+ mov len2=len // for parallelization of error handling
+ mov len3=len
+ tbit.nz p6,p0=len,3
+ ;;
+ EX( .Lexit2, (p6) st8 [buf]=r0,8 ) // at least 8 bytes
+(p6) adds len3=-8,len2
+ tbit.nz p7,p6=len,2
+ ;;
+ EX( .Lexit2, (p7) st4 [buf]=r0,4 ) // at least 4 bytes
+(p7) adds len2=-4,len3
+ tbit.nz p6,p7=len,1
+ ;;
+ EX( .Lexit2, (p6) st2 [buf]=r0,2 ) // at least 2 bytes
+(p6) adds len3=-2,len2
+ tbit.nz p7,p6=len,0
+ ;;
+ EX( .Lexit2, (p7) st1 [buf]=r0 ) // only 1 byte left
+ mov ret0=r0 // success
+ br.ret.sptk.many rp // end of most likely path
+
+ //
+ // Outlined error handling code
+ //
+
+ //
+ // .Lexit3: comes from core loop, need restore pr/lc
+ // len contains bytes left
+ //
+ //
+ // .Lexit2:
+ // if p6 -> coming from st8 or st2 : len2 contains what's left
+ // if p7 -> coming from st4 or st1 : len3 contains what's left
+ // We must restore lc/pr even though might not have been used.
+.Lexit2:
+ .pred.rel "mutex", p6, p7
+(p6) mov len=len2
+(p7) mov len=len3
+ ;;
+ //
+ // .Lexit4: comes from head, need not restore pr/lc
+ // len contains bytes left
+ //
+.Lexit3:
+ mov ret0=len
+ mov ar.lc=saved_lc
+ br.ret.sptk.many rp
+END(__do_clear_user)
diff --git a/xen/arch/x86/usercopy.c b/xen/arch/x86/usercopy.c
index d88e635..58dfc81 100644
--- a/xen/arch/x86/usercopy.c
+++ b/xen/arch/x86/usercopy.c
@@ -110,6 +110,42 @@ copy_to_user(void __user *to, const void *from, unsigned n)
return n;
}
+#define __do_clear_user(addr,size) \
+do { \
+ int __d0; \
+ __asm__ __volatile__( \
+ "0: rep; stosl\n" \
+ " movl %2,%0\n" \
+ "1: rep; stosb\n" \
+ "2:\n" \
+ ".section .fixup,\"ax\"\n" \
+ "3: lea 0(%2,%0,4),%0\n" \
+ " jmp 2b\n" \
+ ".previous\n" \
+ _ASM_EXTABLE(0b,3b) \
+ _ASM_EXTABLE(1b,2b) \
+ : "=&c"(size), "=&D" (__d0) \
+ : "r"(size & 3), "0"(size / 4), "1"(addr), "a"(0)); \
+} while (0)
+
+/**
+ * clear_user: - Zero a block of memory in user space.
+ * @to: Destination address, in user space.
+ * @n: Number of bytes to zero.
+ *
+ * Zero a block of memory in user space.
+ *
+ * Returns number of bytes that could not be cleared.
+ * On success, this will be zero.
+ */
+unsigned long
+clear_user(void __user *to, unsigned n)
+{
+ if ( access_ok(to, n) )
+ __do_clear_user(to, n);
+ return n;
+}
+
/**
* copy_from_user: - Copy a block of data from user space.
* @to: Destination address, in kernel space.
diff --git a/xen/include/asm-ia64/uaccess.h b/xen/include/asm-ia64/uaccess.h
index 32ef415..2ececb1 100644
--- a/xen/include/asm-ia64/uaccess.h
+++ b/xen/include/asm-ia64/uaccess.h
@@ -236,6 +236,18 @@ __copy_from_user (void *to, const void __user *from,
unsigned long count)
__cu_len;
\
})
+extern unsigned long __do_clear_user (void __user * to, unsigned long count);
+
+#define clear_user(to, n) \
+({ \
+ void __user *__cu_to = (to);
\
+ long __cu_len = (n);
\
+
\
+ if (__access_ok(__cu_to))
\
+ __cu_len = __do_clear_user(__cu_to, __cu_len); \
+ __cu_len; \
+})
+
#define copy_from_user(to, from, n)
\
({
\
void *__cu_to = (to);
\
diff --git a/xen/include/asm-x86/uaccess.h b/xen/include/asm-x86/uaccess.h
index e3e541b..d6f4458 100644
--- a/xen/include/asm-x86/uaccess.h
+++ b/xen/include/asm-x86/uaccess.h
@@ -16,6 +16,7 @@
#endif
unsigned long copy_to_user(void *to, const void *from, unsigned len);
+unsigned long clear_user(void *to, unsigned len);
unsigned long copy_from_user(void *to, const void *from, unsigned len);
/* Handles exceptions in both to and from, but doesn't do access_ok */
unsigned long __copy_to_user_ll(void *to, const void *from, unsigned n);
--
1.7.2.5
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