在2024年7月11日七月 下午6:20，Maciej W. Rozycki写道： > 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? Tested flipping nan2008 bits with ieee754=emulated with ptrace, it works on some extent. (flipping the bit to unsupported value immediately triggered emulation). > >> 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. Thanks for the information, I don't have access to those manuals so I was unaware of that. R/W NAN2008 is prohibited by AVP as well. I briefly tested NaN2008 distro on QEMU modified with r/w NaN2008 bits in ieee754= strict mode, it seems working fine. Thanks > > Maciej -- - Jiaxun
