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Re: [Qemu-devel] [RFC PATCH 2/3] tcg: Add support for fence generation i
Re: [Qemu-devel] [RFC PATCH 2/3] tcg: Add support for fence generation in x86 backend
Thu, 26 May 2016 17:09:10 +0100
mu4e 0.9.17; emacs 126.96.36.199
Pranith Kumar <address@hidden> writes:
> Hi Richard,
> Thank you for the helpful comments.
> On Wed, May 25, 2016 at 1:35 PM, Richard Henderson <address@hidden> wrote:
>> On 05/24/2016 10:18 AM, Pranith Kumar wrote:
>>> diff --git a/tcg/i386/tcg-target.h b/tcg/i386/tcg-target.h
>>> index 92be341..93ea42e 100644
>>> --- a/tcg/i386/tcg-target.h
>>> +++ b/tcg/i386/tcg-target.h
>>> @@ -100,6 +100,7 @@ extern bool have_bmi1;
>>> #define TCG_TARGET_HAS_muls2_i32 1
>>> #define TCG_TARGET_HAS_muluh_i32 0
>>> #define TCG_TARGET_HAS_mulsh_i32 0
>>> +#define TCG_TARGET_HAS_fence 1
>> This has to be defined for all hosts.
> OK. I will add an entry in tcg.h with default 0 and override in
> individual architecture once it is implemented.
>>> @@ -347,6 +347,7 @@ static inline int
>>> tcg_target_const_match(tcg_target_long val, TCGType type,
>>> #define OPC_SHRX (0xf7 | P_EXT38 | P_SIMDF2)
>>> #define OPC_TESTL (0x85)
>>> #define OPC_XCHG_ax_r32 (0x90)
>>> +#define OPC_MFENCE (0xAE | P_EXT)
>> Why define OPC_MFENCE if you're not going to use it? Of course, it's not
>> exactly a complete and useful definition, so maybe just delete OPC_MFENCE.
> I want to use OPC_MFENCE instead of hard-coding the value in
> tcg_out_fence(), but as you said the definition is not complete(it
> currently generates only 0x0FAE). I am trying to figure out how to
> generate 0x0FAEF0 using the definition.
I think your going to have to just use tcg_out_fence() and the
tcg_out_opc() does black magic with extra flag bits and assumes it is
encoding rx registers (at least in 64 bit mode).
However I would suggest a comment and maybe breakdown of the different
fence types you can emit. Will pre-P4 processors never need mfences?
>> Also, for 32-bit you need to check for sse2 before outputting this. See
>> also the existing cpuid checks in tcg_target_init and the fallback smp_mb
>> definition for pre-gcc-4.4.
> OK, I'll check the current code and do something similar.