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Re: [Qemu-devel] [PATCH v2] SPARC: Add and use CPU_FEATURE_CASA


From: Sebastian Huber
Subject: Re: [Qemu-devel] [PATCH v2] SPARC: Add and use CPU_FEATURE_CASA
Date: Fri, 14 Feb 2014 09:41:37 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130329 Thunderbird/17.0.5

On 2014-02-13 16:50, Fabien Chouteau wrote:
On 02/13/2014 03:55 PM, Fabien Chouteau wrote:
On 02/13/2014 02:00 PM, Sebastian Huber wrote:
On 2014-02-13 13:01, Fabien Chouteau wrote:
On 02/13/2014 10:52 AM, Sebastian Huber wrote:
The LEON3 processor has support for the CASA instruction which is
normally only available for SPARC V9 processors.  Binutils 2.24
and GCC 4.9 will support this instruction for LEON3.  GCC uses it to
generate C11 atomic operations.

The CAS synthetic instruction uses an ASI of 0x80.  If TARGET_SPARC64 is
not defined use a supervisor data load/store for an ASI of 0x80 in
helper_ld_asi()/helper_st_asi().


Hello Sebastian,

If I understand correctly, the difference with V1 is that ASI 0x80. Why
did you chose Supervisor data access against User data access?

User data access would work also.  I don't have a preference here.

(I cannot
find documentation about 0x80 ASI)

GCC will generate CAS instructions, e.g.

...

In the GNU Binutils you find:

opcodes/sparc-opc.c:{ "cas",    F3(3, 0x3c, 0)|ASI(0x80), F3(~3, ~0x3c, ~0)|ASI(~0x80), 
"[1],2,d", F_ALIAS, 0, v9andleon }, /* casa [rs1]ASI_P,rs2,rd */

This is where the 0x80 comes from.


In some leon3 doc I found this:

62.2.7 Compare and Swap instruction (CASA)
LEON3 implements the SPARC V9 Compare and Swap Alternative (CASA) instruction. 
The CASA
is enabled the interger load delay is set to 1 and the NOTAG generic is 0. The 
CASA operates as
described in the SPARC V9 manual. The instruction is privileged but setting ASI 
= 0xA (user data)
will allow it to be used in user mode.

Which confirm privileged instruction. I will ask our GCC expert if they
know where that 0x80 ASI comes from.


This ASI 0x80 is really defined nowhere in Leon3 not even in the sources :)
Maybe there's a bug in binutils... Did you try to run this program on a real 
board?

Yes, I tested it on a NGMP board with a LEON4 processor (documentation is the same for CAS as in LEON3).

The ASI 0x80 is defined in the SPARC V9 manual, Table 12—Address Space Identifiers (ASIs). Here we have:

0x80, ASI_PRIMARY, Unrestricted access, Primary address space

So should I change it to use User Data Access?

--
Sebastian Huber, embedded brains GmbH

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