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Re: [Qemu-devel] [PATCH v4 5/9] target/mips: Add MXU instruction S8LDD

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v4 5/9] target/mips: Add MXU instruction S8LDD
Date: Wed, 12 Sep 2018 13:19:42 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 08/30/2018 12:30 PM, Craig Janeczek via Qemu-devel wrote:
> +    gen_load_mxu_cr(t0);
> +    tcg_gen_andi_tl(t0, t0, MXUEN);
> +    tcg_gen_brcondi_tl(TCG_COND_NE, t0, MXUEN, l0);

Probably MXUEN should be included in env->hflags, and therefore
tested via ctx->hflags.  (Which also means ending a TB after a
write to MCR, which can change this value).

The documentation says that if MXUEN is unset the result is
"unpredictable".  What does real hardware do?  Does it really
treat the instruction as a nop?  Does it raise an exception?

Otherwise another possibility for "unpredictable" is to simply
execute the instruction.  Which would certainly be the easiest


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