[aarch64-port-dev ] Question about ISB usage in the aarch64 port

[Yangfei (Felix)]

> -----Original Message-----
> From: Andrew Haley [mailto:aph at redhat.com]
> Sent: Thursday, November 21, 2019 6:04 PM
> To: Yangfei (Felix) <felix.yang at huawei.com>;
> aarch64-port-dev at openjdk.java.net
> Subject: Re: [aarch64-port-dev ] Question about ISB usage in the aarch64 port
> 
> On 11/15/19 8:33 AM, Yangfei (Felix) wrote:
> >
> >   The rt_call function is used in generate_native_wrapper to make the
> >   JNI call.
> >   As I didn’t see the barrier for the ppc or arm port.  I would like
> >   to know more details here.  Does anyone still remember?
> 
> What question are you asking? The ISB is there because the callout might run
> concurrently with a safepoint, during which time the code cache may be
> changed by some other thread. While we are in native code safepoints can run
> in other threads without us knowing.

I didn't find this barrier for the ppc or arm port.  
My question: is this necessary to plant a instruction barrier in the same place for those ports?  
Please let me know if I missed anything here.  

> 
> >   Also the ISB is planted only in the else block.  I assume this is
> >   also necessary for the if block.  Correct?
> 
> No. The if block is for calls within the AArch64 Java runtime, so we stay in Java,
> and there shouldn't be any ISB needed. Any part of the Java runtime that loads
> or generates code does its own ISB.

I see.  Does that mean the isb in LIR_Assembler::rt_call should be planted in the else block? 
I mean: 

diff -r dc45ed0ab083 src/hotspot/cpu/aarch64/c1_LIRAssembler_aarch64.cpp
--- a/src/hotspot/cpu/aarch64/c1_LIRAssembler_aarch64.cpp       Wed Nov 13 15:16:45 2019 -0800
+++ b/src/hotspot/cpu/aarch64/c1_LIRAssembler_aarch64.cpp       Thu Nov 21 19:25:00 2019 +0800
@@ -2906,12 +2906,12 @@
   } else {
     __ mov(rscratch1, RuntimeAddress(dest));
     __ blr(rscratch1);
+    __ maybe_isb();
   }

   if (info != NULL) {
     add_call_info_here(info);
   }
-  __ maybe_isb();
 }


Thanks,
Felix