Currently, installation of Debian 12.8 for mipsel fails on machines without an FPU [1]. This is caused by the fact that zstd (which is used for initramfs compression) executes the prefx instruction, which is not emulated properly by the kernel.
The prefx (Prefetch Indexed) instruction fetches data from memory into the cache without any side effects. Though functionally unrelated, it requires an FPU [2].
Bytecode format of this instruction ends on "001111" binary:
(prefx instruction format) & 0x0000003f = 0x0000000f
The code in fpux_emu() runs like so:
#define MIPSInst(x) x #define MIPSInst_FMA_FFMT(x) (MIPSInst(x) & 0x00000007) #define MIPSInst_FUNC(x) (MIPSInst(x) & 0x0000003f) enum cop1x_func { ..., pfetch_op = 0x0f, ... };
...
switch (MIPSInst_FMA_FFMT(ir)) { ...
case 0x3: if (MIPSInst_FUNC(ir) != pfetch_op) return SIGILL;
/* ignore prefx operation */ break;
default: return SIGILL; }
That snippet above contains a logic error and the if (MIPSInst_FUNC(ir) != pfetch_op) comparison always fires.
When MIPSInst_FUNC(ir) is equal to pfetch_op, ir must end on 001111 binary. In this case, MIPSInst_FMA_FFMT(ir) must be equal to 0x7, which does not match that case label.
This causes emulation failure for the prefx instruction. Fix it.
This has been broken by commit 919af8b96c89 ("MIPS: Make definitions of MIPSInst_FMA_{FUNC,FMTM} consistent with MIPS64 manual") which modified the MIPSInst_FMA_FFMT macro without updating the users.
Signed-off-by: Mateusz Jończyk mat.jonczyk@o2.pl Cc: stable@vger.kernel.org # after 3 weeks Cc: Dengcheng Zhu dzhu@wavecomp.com Cc: Thomas Bogendoerfer tsbogend@alpha.franken.de Cc: Ming Wang wangming01@loongson.cn Cc: Tiezhu Yang yangtiezhu@loongson.cn Fixes: 919af8b96c89 ("MIPS: Make definitions of MIPSInst_FMA_{FUNC,FMTM} consistent with MIPS64 manual")
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1091858 [2] MIPS Architecture For Programmers Volume II-A: The MIPS32 Instruction Set
---
Tested in QEMU for mipsel and mips64el. --- arch/mips/math-emu/cp1emu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/mips/math-emu/cp1emu.c b/arch/mips/math-emu/cp1emu.c index 265bc57819df..c89e70df43d8 100644 --- a/arch/mips/math-emu/cp1emu.c +++ b/arch/mips/math-emu/cp1emu.c @@ -1660,7 +1660,7 @@ static int fpux_emu(struct pt_regs *xcp, struct mips_fpu_struct *ctx, break; }
- case 0x3: + case 0x7: if (MIPSInst_FUNC(ir) != pfetch_op) return SIGILL;