linux-kernel - Re: [PATCH v3] MIPS: Implement ieee754 NAN2008 emulation mode

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Message-ID: <alpine.DEB.2.21.2407110315170.38148@angie.orcam.me.uk>
Date: Thu, 11 Jul 2024 11:20:51 +0100 (BST)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Jiaxun Yang <jiaxun.yang@...goat.com>
cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
    Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org, 
    linux-kernel@...r.kernel.org, 
    "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>, 
    Philippe Mathieu-Daudé <philmd@...aro.org>
Subject: Re: [PATCH v3] MIPS: Implement ieee754 NAN2008 emulation mode

On Thu, 11 Jul 2024, Jiaxun Yang wrote:

> >> that's just one case, what about NaN2008 binaries on a legacy MIPS CPU ?
> >
> >  It would be good to check with hard-float QEMU configured for writable 
> > FCSR.NAN2008 (which is one way original code was verified) that things 
> > have not regressed.  And also what happens if once our emulation has 
> > triggered for the unsupported FCSR.NAN2008 mode, an attempt is made to 
> > flip the mode bit via ptrace(2), e.g. under GDB, which I reckon our 
> > emulation permits for non-legacy CPUs (and which I think should not be 
> > allowed under the new setting).
> 
> PTrace is working as expected (reflects emulated value).

 Yes, sure for reads, but how about *writing* to the bit?

> The actual switchable NaN hardware (M5150, P5600) uses a dedicated Config7
> bit rather than writable FCSR.NAN2008 to control NaN2008 mode. This is undocumented
> and not present on some RTL releases. FCSR.NAN2008 is R/O as per The MIPS32 Instruction
> Set Manual. This renders the purposed test pointless.

 Yes, for R6 and arguably R5, but not for R3.  Architecture specification 
revisions 3.50 through 5.02 define FCSR.NAN2008 (and also FCSR.ABS2008) as 
either R/O or R/W, at the implementer's discretion, so it is a conforming 
implementation to have these bits writable and our FPU emulator reflects 
it.  I won't go into the details here as to why the later revisions of the 
specification have been restricted to the R/O implementation only.

 NB architecture specification revisions 3.50 through 5.01 also have the 
FCSR.MAC2008 bit defined, removed altogether later on.

  Maciej