[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH] xen: Introduce cmpxchg64() and guest_cmpxchg64()
Hi Stefano,
On 17/08/2020 23:56, Stefano Stabellini wrote:
On Sat, 15 Aug 2020, Julien Grall wrote:
From: Julien Grall <jgrall@xxxxxxxxxx>
The IOREQ code is using cmpxchg() with 64-bit value. At the moment, this
is x86 code, but there is plan to make it common.
To cater 32-bit arch, introduce two new helpers to deal with 64-bit
cmpxchg.
The Arm 32-bit implementation of cmpxchg64() is based on the __cmpxchg64
in Linux v5.8 (arch/arm/include/asm/cmpxchg.h).
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
---
xen/include/asm-arm/arm32/cmpxchg.h | 68 +++++++++++++++++++++++++++++
xen/include/asm-arm/arm64/cmpxchg.h | 5 +++
xen/include/asm-arm/guest_atomics.h | 22 ++++++++++
xen/include/asm-x86/guest_atomics.h | 2 +
xen/include/asm-x86/x86_64/system.h | 2 +
5 files changed, 99 insertions(+)
diff --git a/xen/include/asm-arm/arm32/cmpxchg.h
b/xen/include/asm-arm/arm32/cmpxchg.h
index 0770f272ee99..5e2fa6ee38a0 100644
--- a/xen/include/asm-arm/arm32/cmpxchg.h
+++ b/xen/include/asm-arm/arm32/cmpxchg.h
@@ -87,6 +87,38 @@ __CMPXCHG_CASE(b, 1)
__CMPXCHG_CASE(h, 2)
__CMPXCHG_CASE( , 4)
+static inline bool __cmpxchg_case_8(volatile uint64_t *ptr,
+ uint64_t *old,
+ uint64_t new,
+ bool timeout,
+ unsigned int max_try)
+{
+ uint64_t oldval;
+ uint64_t res;
+
+ do {
+ asm volatile(
+ " ldrexd %1, %H1, [%3]\n"
+ " teq %1, %4\n"
+ " teqeq %H1, %H4\n"
+ " movne %0, #0\n"
+ " movne %H0, #0\n"
+ " bne 2f\n"
+ " strexd %0, %5, %H5, [%3]\n"
+ " teq %0, #0\n"
Apologies if I am misreading this code, but this last "teq" instruction
doesn't seem to be useful?
Urgh, I forgot to remove it. The Linux version is looping in assembly
but I had to convert to C in order to cater the timeout.
I will remove it in the next version.
+ "2:"
+ : "=&r" (res), "=&r" (oldval), "+Qo" (*ptr)
^ not used ?
+ : "r" (ptr), "r" (*old), "r" (new)
+ : "memory", "cc");
+ if (!res)
+ break;
+ } while (!timeout || ((--max_try) > 0));
+
+ *old = oldval;
+
+ return !res;
+}
+
static always_inline bool __int_cmpxchg(volatile void *ptr, unsigned long
*old,
unsigned long new, int size,
bool timeout, unsigned int max_try)
@@ -156,6 +188,30 @@ static always_inline bool __cmpxchg_mb_timeout(volatile
void *ptr,
return ret;
}
+/*
+ * The helper may fail to update the memory if the action takes too long.
+ *
+ * @old: On call the value pointed contains the expected old value. It will be
+ * updated to the actual old value.
+ * @max_try: Maximum number of iterations
+ *
+ * The helper will return true when the update has succeeded (i.e no
+ * timeout) and false if the update has failed.
+ */
+static always_inline bool __cmpxchg64_mb_timeout(volatile uint64_t *ptr,
+ uint64_t *old,
+ uint64_t new,
+ unsigned int max_try)
+{
+ bool ret;
+
+ smp_mb();
+ ret = __cmpxchg_case_8(ptr, old, new, true, max_try);
+ smp_mb();
+
+ return ret;
+}
+
#define cmpxchg(ptr,o,n) \
((__typeof__(*(ptr)))__cmpxchg_mb((ptr), \
(unsigned long)(o), \
@@ -167,6 +223,18 @@ static always_inline bool __cmpxchg_mb_timeout(volatile
void *ptr,
(unsigned long)(o), \
(unsigned long)(n), \
sizeof(*(ptr))))
+
+static inline uint64_t cmpxchg64(volatile uint64_t *ptr,
+ uint64_t old,
+ uint64_t new)
+{
+ smp_mb();
I was looking at the existing code I noticed that we don't have a
corresponding smp_mb(); in this position. Is it needed here because of
the 64bit-ness?
We have barriers also in the existing. The code can be a bit confusing
because __cmpxchg() refers to a local cmpxchg.
In our case, the corresponding version if __cmpxchg_mb().
To be honest, the existing naming is a bit confusing. I am thinking to
drop cmpxcgh_local() completely as this is not used. This would also
make the cod easier to read. What do you think?
+ if (!__cmpxchg_case_8(ptr, &old, new, false, 0))
+ ASSERT_UNREACHABLE();
And I forgot the smp_mb() here :(.
+
+ return old;
+}
+
#endif
/*
* Local variables:
diff --git a/xen/include/asm-arm/arm64/cmpxchg.h
b/xen/include/asm-arm/arm64/cmpxchg.h
index fc5c60f0bd74..de9cd0ee2b07 100644
--- a/xen/include/asm-arm/arm64/cmpxchg.h
+++ b/xen/include/asm-arm/arm64/cmpxchg.h
@@ -187,6 +187,11 @@ static always_inline bool __cmpxchg_mb_timeout(volatile
void *ptr,
__ret; \
})
+#define cmpxchg64(ptr, o, n) cmpxchg(ptr, o, n)
+
+#define __cmpxchg64_mb_timeout(ptr, old, new, max_try) \
+ __cmpxchg_mb_timeout(ptr, old, new, 8, max_try)
+
#endif
/*
* Local variables:
diff --git a/xen/include/asm-arm/guest_atomics.h
b/xen/include/asm-arm/guest_atomics.h
index af27cc627bf3..28ce402bea79 100644
--- a/xen/include/asm-arm/guest_atomics.h
+++ b/xen/include/asm-arm/guest_atomics.h
@@ -115,6 +115,28 @@ static inline unsigned long __guest_cmpxchg(struct domain
*d,
(unsigned long)(n),\
sizeof (*(ptr))))
+static inline uint64_t guest_cmpxchg64(struct domain *d,
+ volatile uint64_t *ptr,
+ uint64_t old,
+ uint64_t new)
+{
+ uint64_t oldval = old;
+
+ perfc_incr(atomics_guest);
+
+ if ( __cmpxchg64_mb_timeout(ptr, &oldval, new,
+ this_cpu(guest_safe_atomic_max)) )
+ return oldval;
+
+ perfc_incr(atomics_guest_paused);
+
+ domain_pause_nosync(d);
+ oldval = cmpxchg64(ptr, old, new);
+ domain_unpause(d);
+
+ return oldval;
+}
+
#endif /* _ARM_GUEST_ATOMICS_H */
/*
* Local variables:
diff --git a/xen/include/asm-x86/guest_atomics.h
b/xen/include/asm-x86/guest_atomics.h
index 029417c8ffc1..f4de9d3631ff 100644
--- a/xen/include/asm-x86/guest_atomics.h
+++ b/xen/include/asm-x86/guest_atomics.h
@@ -20,6 +20,8 @@
((void)(d), test_and_change_bit(nr, p))
#define guest_cmpxchg(d, ptr, o, n) ((void)(d), cmpxchg(ptr, o, n))
+#define guest_cmpxchg64(d, ptr, o, n) ((void)(d), cmpxchg64(ptr, o, n))
+
#endif /* _X86_GUEST_ATOMICS_H */
/*
diff --git a/xen/include/asm-x86/x86_64/system.h
b/xen/include/asm-x86/x86_64/system.h
index f471859c19cc..c1b16105e9f2 100644
--- a/xen/include/asm-x86/x86_64/system.h
+++ b/xen/include/asm-x86/x86_64/system.h
@@ -5,6 +5,8 @@
((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o), \
(unsigned long)(n),sizeof(*(ptr))))
+#define cmpxchg64(ptr, o, n) cmpxchg(ptr, o, n)
+
/*
* Atomic 16 bytes compare and exchange. Compare OLD with MEM, if
* identical, store NEW in MEM. Return the initial value in MEM.
--
2.17.1
Cheers,
--
Julien Grall
|
