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

[Sebastian Huber]

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.

        .file   "ticket.c"
        .section        ".text"
        .align 4
        .global acquire
        .type   acquire, #function
        .proc   020
acquire:
        ld      [%o0], %g1
        mov     %g1, %g2
.LL7:
        add     %g1, 1, %g1
        cas     [%o0], %g2, %g1
        cmp     %g1, %g2
        bne,a   .LL7
         mov    %g1, %g2
        add     %o0, 4, %o0
.LL4:
        ld      [%o0], %g1
        cmp     %g1, %g2
        bne     .LL4
         nop
        jmp     %o7+8
         nop
        .size   acquire, .-acquire
        .align 4
        .global release
        .type   release, #function
        .proc   020
release:
        ld      [%o0+4], %g1
        add     %g1, 1, %g1
        st      %g1, [%o0+4]
        jmp     %o7+8
         nop
        .size   release, .-release
        .ident  "GCC: (GNU) 4.9.0 20130917 (experimental)"

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.

--
Sebastian Huber, embedded brains GmbH

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