[tip: x86/cpu] x86/clear_user: Make it faster

From: tip-bot2 for Borislav Petkov
Date: Thu Aug 18 2022 - 06:44:49 EST

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

Commit-ID: 0db7058e8e23e6bbab1b4747ecabd1784c34f50b
Gitweb: https://git.kernel.org/tip/0db7058e8e23e6bbab1b4747ecabd1784c34f50b
Author: Borislav Petkov <bp@xxxxxxx>
AuthorDate: Tue, 24 May 2022 11:01:18 +02:00
Committer: Borislav Petkov <bp@xxxxxxx>
CommitterDate: Thu, 18 Aug 2022 12:36:42 +02:00

x86/clear_user: Make it faster

Based on a patch by Mark Hemment <markhemm@xxxxxxxxxxxxxx> and
incorporating very sane suggestions from Linus.

The point here is to have the default case with FSRM - which is supposed
to be the majority of x86 hw out there - if not now then soon - be
directly inlined into the instruction stream so that no function call
overhead is taking place.

Drop the early clobbers from the @size and @addr operands as those are
not needed anymore since we have single instruction alternatives.

The benchmarks I ran would show very small improvements and a PF
benchmark would even show weird things like slowdowns with higher core
counts.

So for a ~6m running the git test suite, the function gets called under
700K times, all from padzero():

<...>-2536 [006] ..... 261.208801: padzero: to: 0x55b0663ed214, size: 3564, cycles: 21900
<...>-2536 [006] ..... 261.208819: padzero: to: 0x7f061adca078, size: 3976, cycles: 17160
<...>-2537 [008] ..... 261.211027: padzero: to: 0x5572d019e240, size: 3520, cycles: 23850
<...>-2537 [008] ..... 261.211049: padzero: to: 0x7f1288dc9078, size: 3976, cycles: 15900
...

which is around 1%-ish of the total time and which is consistent with
the benchmark numbers.

So Mel gave me the idea to simply measure how fast the function becomes.
I.e.:

start = rdtsc_ordered();
ret = __clear_user(to, n);
end = rdtsc_ordered();

Computing the mean average of all the samples collected during the test
suite run then shows some improvement:

clear_user_original:
Amean: 9219.71 (Sum: 6340154910, samples: 687674)

fsrm:
Amean: 8030.63 (Sum: 5522277720, samples: 687652)

That's on Zen3.

The situation looks a lot more confusing on Intel:

Icelake:

clear_user_original:
Amean: 19679.4 (Sum: 13652560764, samples: 693750)
Amean: 19743.7 (Sum: 13693470604, samples: 693562)

(I ran it twice just to be sure.)

ERMS:
Amean: 20374.3 (Sum: 13910601024, samples: 682752)
Amean: 20453.7 (Sum: 14186223606, samples: 693576)

FSRM:
Amean: 20458.2 (Sum: 13918381386, sample s: 680331)

The original microbenchmark which people were complaining about:

for i in $(seq 1 10); do dd if=/dev/zero of=/dev/null bs=1M status=progress count=65536; done 2>&1 | grep copied
32207011840 bytes (32 GB, 30 GiB) copied, 1 s, 32.2 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.93069 s, 35.6 GB/s
37597741056 bytes (38 GB, 35 GiB) copied, 1 s, 37.6 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.78017 s, 38.6 GB/s
62020124672 bytes (62 GB, 58 GiB) copied, 2 s, 31.0 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 2.13716 s, 32.2 GB/s
60010004480 bytes (60 GB, 56 GiB) copied, 1 s, 60.0 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.14129 s, 60.2 GB/s
53212086272 bytes (53 GB, 50 GiB) copied, 1 s, 53.2 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.28398 s, 53.5 GB/s
55698259968 bytes (56 GB, 52 GiB) copied, 1 s, 55.7 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.22507 s, 56.1 GB/s
55306092544 bytes (55 GB, 52 GiB) copied, 1 s, 55.3 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.23647 s, 55.6 GB/s
54387539968 bytes (54 GB, 51 GiB) copied, 1 s, 54.4 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.25693 s, 54.7 GB/s
50566529024 bytes (51 GB, 47 GiB) copied, 1 s, 50.6 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.35096 s, 50.9 GB/s
58308165632 bytes (58 GB, 54 GiB) copied, 1 s, 58.3 GB/s
68719476736 bytes (69 GB, 64 GiB) copied, 1.17394 s, 58.5 GB/s

Now the same thing with smaller buffers:

for i in $(seq 1 10); do dd if=/dev/zero of=/dev/null bs=1M status=progress count=8192; done 2>&1 | grep copied
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.28485 s, 30.2 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.276112 s, 31.1 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.29136 s, 29.5 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.283803 s, 30.3 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.306503 s, 28.0 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.349169 s, 24.6 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.276912 s, 31.0 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.265356 s, 32.4 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.28464 s, 30.2 GB/s
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 0.242998 s, 35.3 GB/s

is also not conclusive because it all depends on the buffer sizes,
their alignments and when the microcode detects that cachelines can be
aggregated properly and copied in bigger sizes.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lore.kernel.org/r/CAHk-=wh=Mu_EYhtOmPn6AxoQZyEh-4fo2Zx3G7rBv1g7vwoKiw@xxxxxxxxxxxxxx
---
arch/x86/include/asm/uaccess.h | 5 +-
arch/x86/include/asm/uaccess_64.h | 45 +++++++++-
arch/x86/lib/clear_page_64.S | 138 +++++++++++++++++++++++++++++-
arch/x86/lib/usercopy_64.c | 40 +--------
tools/objtool/check.c | 3 +-
5 files changed, 188 insertions(+), 43 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 913e593..c462079 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -502,9 +502,6 @@ strncpy_from_user(char *dst, const char __user *src, long count);

extern __must_check long strnlen_user(const char __user *str, long n);

-unsigned long __must_check clear_user(void __user *mem, unsigned long len);
-unsigned long __must_check __clear_user(void __user *mem, unsigned long len);
-
#ifdef CONFIG_ARCH_HAS_COPY_MC
unsigned long __must_check
copy_mc_to_kernel(void *to, const void *from, unsigned len);
@@ -526,6 +523,8 @@ extern struct movsl_mask {
#define ARCH_HAS_NOCACHE_UACCESS 1

#ifdef CONFIG_X86_32
+unsigned long __must_check clear_user(void __user *mem, unsigned long len);
+unsigned long __must_check __clear_user(void __user *mem, unsigned long len);
# include <asm/uaccess_32.h>
#else
# include <asm/uaccess_64.h>
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index 45697e0..d13d71a 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -79,4 +79,49 @@ __copy_from_user_flushcache(void *dst, const void __user *src, unsigned size)
kasan_check_write(dst, size);
return __copy_user_flushcache(dst, src, size);
}
+
+/*
+ * Zero Userspace.
+ */
+
+__must_check unsigned long
+clear_user_original(void __user *addr, unsigned long len);
+__must_check unsigned long
+clear_user_rep_good(void __user *addr, unsigned long len);
+__must_check unsigned long
+clear_user_erms(void __user *addr, unsigned long len);
+
+static __always_inline __must_check unsigned long __clear_user(void __user *addr, unsigned long size)
+{
+ might_fault();
+ stac();
+
+ /*
+ * No memory constraint because it doesn't change any memory gcc
+ * knows about.
+ */
+ asm volatile(
+ "1:\n\t"
+ ALTERNATIVE_3("rep stosb",
+ "call clear_user_erms", ALT_NOT(X86_FEATURE_FSRM),
+ "call clear_user_rep_good", ALT_NOT(X86_FEATURE_ERMS),
+ "call clear_user_original", ALT_NOT(X86_FEATURE_REP_GOOD))
+ "2:\n"
+ _ASM_EXTABLE_UA(1b, 2b)
+ : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT
+ : "a" (0)
+ /* rep_good clobbers %rdx */
+ : "rdx");
+
+ clac();
+
+ return size;
+}
+
+static __always_inline unsigned long clear_user(void __user *to, unsigned long n)
+{
+ if (access_ok(to, n))
+ return __clear_user(to, n);
+ return n;
+}
#endif /* _ASM_X86_UACCESS_64_H */
diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
index fe59b8a..ecbfb4d 100644
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -1,5 +1,6 @@
/* SPDX-License-Identifier: GPL-2.0-only */
#include <linux/linkage.h>
+#include <asm/asm.h>
#include <asm/export.h>

/*
@@ -50,3 +51,140 @@ SYM_FUNC_START(clear_page_erms)
RET
SYM_FUNC_END(clear_page_erms)
EXPORT_SYMBOL_GPL(clear_page_erms)
+
+/*
+ * Default clear user-space.
+ * Input:
+ * rdi destination
+ * rcx count
+ *
+ * Output:
+ * rcx: uncleared bytes or 0 if successful.
+ */
+SYM_FUNC_START(clear_user_original)
+ /*
+ * Copy only the lower 32 bits of size as that is enough to handle the rest bytes,
+ * i.e., no need for a 'q' suffix and thus a REX prefix.
+ */
+ mov %ecx,%eax
+ shr $3,%rcx
+ jz .Lrest_bytes
+
+ # do the qwords first
+ .p2align 4
+.Lqwords:
+ movq $0,(%rdi)
+ lea 8(%rdi),%rdi
+ dec %rcx
+ jnz .Lqwords
+
+.Lrest_bytes:
+ and $7, %eax
+ jz .Lexit
+
+ # now do the rest bytes
+.Lbytes:
+ movb $0,(%rdi)
+ inc %rdi
+ dec %eax
+ jnz .Lbytes
+
+.Lexit:
+ /*
+ * %rax still needs to be cleared in the exception case because this function is called
+ * from inline asm and the compiler expects %rax to be zero when exiting the inline asm,
+ * in case it might reuse it somewhere.
+ */
+ xor %eax,%eax
+ RET
+
+.Lqwords_exception:
+ # convert remaining qwords back into bytes to return to caller
+ shl $3, %rcx
+ and $7, %eax
+ add %rax,%rcx
+ jmp .Lexit
+
+.Lbytes_exception:
+ mov %eax,%ecx
+ jmp .Lexit
+
+ _ASM_EXTABLE_UA(.Lqwords, .Lqwords_exception)
+ _ASM_EXTABLE_UA(.Lbytes, .Lbytes_exception)
+SYM_FUNC_END(clear_user_original)
+EXPORT_SYMBOL(clear_user_original)
+
+/*
+ * Alternative clear user-space when CPU feature X86_FEATURE_REP_GOOD is
+ * present.
+ * Input:
+ * rdi destination
+ * rcx count
+ *
+ * Output:
+ * rcx: uncleared bytes or 0 if successful.
+ */
+SYM_FUNC_START(clear_user_rep_good)
+ # call the original thing for less than a cacheline
+ cmp $64, %rcx
+ jb clear_user_original
+
+.Lprep:
+ # copy lower 32-bits for rest bytes
+ mov %ecx, %edx
+ shr $3, %rcx
+ jz .Lrep_good_rest_bytes
+
+.Lrep_good_qwords:
+ rep stosq
+
+.Lrep_good_rest_bytes:
+ and $7, %edx
+ jz .Lrep_good_exit
+
+.Lrep_good_bytes:
+ mov %edx, %ecx
+ rep stosb
+
+.Lrep_good_exit:
+ # see .Lexit comment above
+ xor %eax, %eax
+ RET
+
+.Lrep_good_qwords_exception:
+ # convert remaining qwords back into bytes to return to caller
+ shl $3, %rcx
+ and $7, %edx
+ add %rdx, %rcx
+ jmp .Lrep_good_exit
+
+ _ASM_EXTABLE_UA(.Lrep_good_qwords, .Lrep_good_qwords_exception)
+ _ASM_EXTABLE_UA(.Lrep_good_bytes, .Lrep_good_exit)
+SYM_FUNC_END(clear_user_rep_good)
+EXPORT_SYMBOL(clear_user_rep_good)
+
+/*
+ * Alternative clear user-space when CPU feature X86_FEATURE_ERMS is present.
+ * Input:
+ * rdi destination
+ * rcx count
+ *
+ * Output:
+ * rcx: uncleared bytes or 0 if successful.
+ *
+ */
+SYM_FUNC_START(clear_user_erms)
+ # call the original thing for less than a cacheline
+ cmp $64, %rcx
+ jb clear_user_original
+
+.Lerms_bytes:
+ rep stosb
+
+.Lerms_exit:
+ xorl %eax,%eax
+ RET
+
+ _ASM_EXTABLE_UA(.Lerms_bytes, .Lerms_exit)
+SYM_FUNC_END(clear_user_erms)
+EXPORT_SYMBOL(clear_user_erms)
diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index 0ae6cf8..6c1f8ac 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -14,46 +14,6 @@
* Zero Userspace
*/

-unsigned long __clear_user(void __user *addr, unsigned long size)
-{
- long __d0;
- might_fault();
- /* no memory constraint because it doesn't change any memory gcc knows
- about */
- stac();
- asm volatile(
- " testq %[size8],%[size8]\n"
- " jz 4f\n"
- " .align 16\n"
- "0: movq $0,(%[dst])\n"
- " addq $8,%[dst]\n"
- " decl %%ecx ; jnz 0b\n"
- "4: movq %[size1],%%rcx\n"
- " testl %%ecx,%%ecx\n"
- " jz 2f\n"
- "1: movb $0,(%[dst])\n"
- " incq %[dst]\n"
- " decl %%ecx ; jnz 1b\n"
- "2:\n"
-
- _ASM_EXTABLE_TYPE_REG(0b, 2b, EX_TYPE_UCOPY_LEN8, %[size1])
- _ASM_EXTABLE_UA(1b, 2b)
-
- : [size8] "=&c"(size), [dst] "=&D" (__d0)
- : [size1] "r"(size & 7), "[size8]" (size / 8), "[dst]"(addr));
- clac();
- return size;
-}
-EXPORT_SYMBOL(__clear_user);
-
-unsigned long clear_user(void __user *to, unsigned long n)
-{
- if (access_ok(to, n))
- return __clear_user(to, n);
- return n;
-}
-EXPORT_SYMBOL(clear_user);
-
#ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
/**
* clean_cache_range - write back a cache range with CLWB
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0cec74d..4b2e117 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1071,6 +1071,9 @@ static const char *uaccess_safe_builtin[] = {
"copy_mc_fragile_handle_tail",
"copy_mc_enhanced_fast_string",
"ftrace_likely_update", /* CONFIG_TRACE_BRANCH_PROFILING */
+ "clear_user_erms",
+ "clear_user_rep_good",
+ "clear_user_original",
NULL
};

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