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Re: [qemu-s390x] [Qemu-devel] [PATCH v1 08/15] s390x/tcg: Handle SET FPC

From: Richard Henderson
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH v1 08/15] s390x/tcg: Handle SET FPC AND LOAD FPC 3-bit BFP rounding modes
Date: Tue, 12 Feb 2019 11:07:44 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 2/12/19 3:03 AM, David Hildenbrand wrote:
> We already forward the 3 bits correctly in the translation functions. We
> also have to handle them properly and check for specification
> exceptions.
> Setting an invalid rounding mode (BFP only, all DFP rounding modes)
> results in a specification exception. Setting unassigned bits in the
> fpc, results in a specification exception.
> This fixes LOAD FPC (AND SIGNAL), SET FPC (AND SIGNAL). Also for,
> SET BFP ROUNDING MODE, 3-bit rounding mode is now explicitly checked.
> Notes:
> 1. Use "float_round_to_zero" for now to handle "Round to prepare for
>    shorter precision". Looking at the PoP "Summary of Rounding and Range
>    Actions" for BFP. They differ when it comes to tiny values.
> 2. TCG_CALL_NO_WG is required for sfpc handler, as we now inject
>    exceptions.
> We won't be modeling abscence of the "floating-point extension facility"
> for now, not necessary as most take the facility for granted without
> checking.
> z14 PoP, 9-23, "LOAD FPC"
>     When the floating-point extension facility is
>     installed, bits 29-31 of the second operand must
>     specify a valid BFP rounding mode and bits 6-7,
>     14-15, 24, and 28 must be zero; otherwise, a
>     specification exception is recognized.

Reviewed-by: Richard Henderson <address@hidden>

> +    /*
> +     * FIXME: we actually want something like round_to_odd, but that does not
> +     * support all data types yet.
> +     */
> +    float_round_to_zero,

Yes, you want round_to_odd.  I suppose that's not valid for float128 right now?


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