From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36925) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bPBnr-0006hh-Np for qemu-devel@nongnu.org; Mon, 18 Jul 2016 12:54:01 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bPBnm-0001vB-I9 for qemu-devel@nongnu.org; Mon, 18 Jul 2016 12:53:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42591) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bPBnm-0001ut-9Q for qemu-devel@nongnu.org; Mon, 18 Jul 2016 12:53:54 -0400 References: <1468851450-9863-1-git-send-email-pbonzini@redhat.com> <578D0938.2050004@gmail.com> From: Paolo Bonzini Message-ID: Date: Mon, 18 Jul 2016 18:53:50 +0200 MIME-Version: 1.0 In-Reply-To: <578D0938.2050004@gmail.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Sergey Fedorov , qemu-devel@nongnu.org Cc: sergey.fedorov@linaro.org, alex.bennee@linaro.org On 18/07/2016 18:52, Sergey Fedorov wrote: > So how are we going to use them? Instead of atomic_read/atomic_set when marking invalid TBs. diff --git a/cpu-exec.c b/cpu-exec.c index fd43de8..1275f3d 100644 --- a/cpu-exec.c +++ b/cpu-exec.c @@ -292,10 +292,10 @@ static inline TranslationBlock *tb_find(CPUState *cpu, always be the same before a given translated block is executed. */ cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags); - tb = atomic_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]); - if (unlikely(!tb || atomic_read(&tb->pc) != pc || - atomic_read(&tb->cs_base) != cs_base || - atomic_read(&tb->flags) != flags)) { + tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]); + if (unlikely(!tb || volatile_read(&tb->pc) != pc || + volatile_read(&tb->cs_base) != cs_base || + volatile_read(&tb->flags) != flags)) { tb = tb_htable_lookup(cpu, pc, cs_base, flags); if (!tb) { diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h index 8f0afcd..35e963b 100644 --- a/include/exec/exec-all.h +++ b/include/exec/exec-all.h @@ -262,9 +262,9 @@ static inline void tb_mark_invalid(TranslationBlock *tb) uint32_t flags = 0; cpu_get_invalid_tb_cpu_state(&pc, &cs_base, &flags); - atomic_set(&tb->pc, pc); - atomic_set(&tb->cs_base, cs_base); - atomic_set(&tb->flags, flags); + volatile_set(&tb->pc, pc); + volatile_set(&tb->cs_base, cs_base); + volatile_set(&tb->flags, flags); } static inline bool tb_is_invalid(TranslationBlock *tb) Thanks, Paolo > Thanks, > Sergey > > On 18/07/16 17:17, Paolo Bonzini wrote: >> Signed-off-by: Paolo Bonzini >> --- >> docs/atomics.txt | 19 ++++++++++++++++--- >> include/qemu/atomic.h | 17 +++++++++++++++++ >> 2 files changed, 33 insertions(+), 3 deletions(-) >> >> diff --git a/docs/atomics.txt b/docs/atomics.txt >> index c95950b..1f21d2e 100644 >> --- a/docs/atomics.txt >> +++ b/docs/atomics.txt >> @@ -123,6 +123,14 @@ to do so, because it tells readers which variables are shared with >> other threads, and which are local to the current thread or protected >> by other, more mundane means. >> >> +atomic_read() and atomic_set() only support accesses as large as a >> +pointer. If you need to access variables larger than a pointer you >> +can use volatile_read() and volatile_set(), but be careful: these always >> +use volatile accesses, and 64-bit volatile accesses are not atomic on >> +several 32-bit processors such as ARMv7. In other words, volatile_read >> +and volatile_set only provide "safe register" semantics when applied to >> +64-bit variables. >> + >> Memory barriers control the order of references to shared memory. >> They come in four kinds: >> >> @@ -335,11 +343,16 @@ and memory barriers, and the equivalents in QEMU: >> Both semantics prevent the compiler from doing certain transformations; >> the difference is that atomic accesses are guaranteed to be atomic, >> while volatile accesses aren't. Thus, in the volatile case we just cross >> - our fingers hoping that the compiler will generate atomic accesses, >> - since we assume the variables passed are machine-word sized and >> - properly aligned. >> + our fingers hoping that the compiler and processor will provide atomic >> + accesses, since we assume the variables passed are machine-word sized >> + and properly aligned. >> + >> No barriers are implied by atomic_read/set in either Linux or QEMU. >> >> +- volatile_read and volatile_set are equivalent to ACCESS_ONCE in Linux. >> + No barriers are implied by volatile_read/set in QEMU, nor by >> + ACCESS_ONCE in Linux. >> + >> - atomic read-modify-write operations in Linux are of three kinds: >> >> atomic_OP returns void >> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h >> index 7e13fca..8409bdb 100644 >> --- a/include/qemu/atomic.h >> +++ b/include/qemu/atomic.h >> @@ -18,6 +18,12 @@ >> /* Compiler barrier */ >> #define barrier() ({ asm volatile("" ::: "memory"); (void)0; }) >> >> +/* These will only be atomic if the processor does the fetch or store >> + * in a single issue memory operation >> + */ >> +#define volatile_read(ptr) (*(__typeof__(*ptr) volatile*) (ptr)) >> +#define volatile_set(ptr, i) ((*(__typeof__(*ptr) volatile*) (ptr)) = (i)) >> + >> #ifdef __ATOMIC_RELAXED >> /* For C11 atomic ops */ >> >> @@ -260,6 +266,17 @@ >> */ >> #define atomic_read(ptr) (*(__typeof__(*ptr) volatile*) (ptr)) >> #define atomic_set(ptr, i) ((*(__typeof__(*ptr) volatile*) (ptr)) = (i)) >> +#define atomic_read(ptr) \ >> + ({ \ >> + QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \ >> + volatile_read(ptr); \ >> + }) >> + >> +#define atomic_set(ptr, i) do { \ >> + QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \ >> + volatile_set(ptr, i); \ >> +} while(0) >> + >> >> /** >> * atomic_rcu_read - reads a RCU-protected pointer to a local variable >