Re: [Qemu-devel] [PATCH v2 10/11] tcg/mips: Make direct jump patching thread-safe

[Aurelien Jarno]

On 2016-04-22 18:47, Aurelien Jarno wrote:
> On 2016-04-22 19:08, Sergey Fedorov wrote:
> > From: Sergey Fedorov <address@hidden>
> > 
> > Ensure direct jump patching in MIPS is atomic by using
> > atomic_read()/atomic_set() for code patching.
> > 
> > Signed-off-by: Sergey Fedorov <address@hidden>
> > Signed-off-by: Sergey Fedorov <address@hidden>
> > ---
> > 
> > Changes in v2:
> >  * s/atomic_write/atomic_set/
> > 
> >  tcg/mips/tcg-target.inc.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tcg/mips/tcg-target.inc.c b/tcg/mips/tcg-target.inc.c
> > index 682e19897db0..cefc0398018a 100644
> > --- a/tcg/mips/tcg-target.inc.c
> > +++ b/tcg/mips/tcg-target.inc.c
> > @@ -1886,6 +1886,7 @@ static void tcg_target_init(TCGContext *s)
> >  void tb_set_jmp_target1(uintptr_t jmp_addr, uintptr_t addr)
> >  {
> >      uint32_t *ptr = (uint32_t *)jmp_addr;
> > -    *ptr = deposit32(*ptr, 0, 26, addr >> 2);
> > +    uint32_t insn = atomic_read(ptr);
> > +    atomic_set(ptr, deposit32(insn, 0, 26, addr >> 2));
> >      flush_icache_range(jmp_addr, jmp_addr + 4);
> 
> Does it really make sense to read and write the value atomically? The
> resulting operation is still not atomic, something can happen in
> between.

Hmm, thinking more about that, given the only instruction used is "J",
we don't have to read the value, patch it and write it. We can directly
use something like (untested):

    atomic_set(ptr, (0x02 << 26) | (addr >> 2));

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net