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Re: [Qemu-devel] [RFC v3 PATCH 14/14] target-i386: Generate fences for x

From: Richard Henderson
Subject: Re: [Qemu-devel] [RFC v3 PATCH 14/14] target-i386: Generate fences for x86
Date: Tue, 21 Jun 2016 08:57:27 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 06/21/2016 12:28 AM, Paolo Bonzini wrote:

On 18/06/2016 06:03, Pranith Kumar wrote:
Signed-off-by: Pranith Kumar <address@hidden>
 target-i386/translate.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/target-i386/translate.c b/target-i386/translate.c
index bf33e6b..32b0f5c 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -8012,13 +8012,17 @@ static target_ulong disas_insn(CPUX86State *env, 
DisasContext *s,
                 || (prefixes & PREFIX_LOCK)) {
                 goto illegal_op;
+            tcg_gen_mb(TCG_MO_ST_ST | TCG_BAR_SC);
         case 0xe8 ... 0xef: /* lfence */
+            tcg_gen_mb(TCG_MO_LD_LD | TCG_BAR_SC);
+            break;

These are unnecessary.  On the other hand, _each and every load_ must be
followed by a LD_LD | LD_ST barrier, and each and every store must be
preceded by a LD_ST | ST_ST barrier.

They're not unnecessary if we (1) add those barriers for normal loads and stores and (2) omit them from the non-temporal loads and stores.

In the meantime they're good documentation.


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