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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v2 2/2] xen: Introduce cmpxchg64() and guest_cmpxchg64()
On Fri, 11 Sep 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).
>
> Note that only guest_cmpxchg64() is introduced on x86 so far.
>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> ---
>
> I have looked at whether we can extend cmpxchg() to support 64-bit on
> arm32 bit. However the resulting code is much worse as the compiler will
> use the stack more often to spill. Therefore the introduction of
> cmpxchg64() is better option.
>
> Changes in v2:
> - Remove extra teq in the arm32 cmpxchg implementation
> - Don't define cmpxchg64() on x86
> - Drop _mb from the helpers name
> - Add missing barrier in the arm32 implementation
> ---
> 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 | 1 +
> 4 files changed, 96 insertions(+)
>
> diff --git a/xen/include/asm-arm/arm32/cmpxchg.h
> b/xen/include/asm-arm/arm32/cmpxchg.h
> index 3ef1e5c63276..b0bd1d8b685e 100644
> --- a/xen/include/asm-arm/arm32/cmpxchg.h
> +++ b/xen/include/asm-arm/arm32/cmpxchg.h
> @@ -87,6 +87,37 @@ __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"
> + "2:"
> + : "=&r" (res), "=&r" (oldval), "+Qo" (*ptr)
> + : "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)
> @@ -145,11 +176,48 @@ static always_inline bool __cmpxchg_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_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((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();
> + if (!__cmpxchg_case_8(ptr, &old, new, false, 0))
> + ASSERT_UNREACHABLE();
> + smp_mb();
> +
> + return old;
> +}
> +
> #endif
> /*
> * Local variables:
> diff --git a/xen/include/asm-arm/arm64/cmpxchg.h
> b/xen/include/asm-arm/arm64/cmpxchg.h
> index f4a8c1ccba80..10e4edc022b7 100644
> --- a/xen/include/asm-arm/arm64/cmpxchg.h
> +++ b/xen/include/asm-arm/arm64/cmpxchg.h
> @@ -167,6 +167,11 @@ static always_inline bool __cmpxchg_timeout(volatile
> void *ptr,
> __ret; \
> })
>
> +#define cmpxchg64(ptr, o, n) cmpxchg(ptr, o, n)
> +
> +#define __cmpxchg64_timeout(ptr, old, new, max_try) \
> + __cmpxchg_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 20347849e56c..9e2e96d4ff72 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_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..87e7075b7623 100644
> --- a/xen/include/asm-x86/guest_atomics.h
> +++ b/xen/include/asm-x86/guest_atomics.h
> @@ -20,6 +20,7 @@
> ((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), cmpxchg(ptr, o, n))
>
> #endif /* _X86_GUEST_ATOMICS_H */
> /*
> --
> 2.17.1
>
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