In this issue
https://github.com/cloudius-systems/osv/issues/827#issuecomment-271140573 Nadav
points out that for whatever reason we are NOT initializing FPU control set
in init_fpu(). I think that by default FNINIT operation does set it to this
value - *0x037f *- which per this
(https://sites.uclouvain.be/SystInfo/usr/include/fpu_control.h.html) sets
double extended precision, nearest rounding control and masks all FPU
exceptions.
But I think it would not hurt to set this explicitly (I looked up the
FreeBSD code which does this).
diff --git a/arch/x64/processor.hh b/arch/x64/processor.hh
index a26fb028..142742ca 100644
--- a/arch/x64/processor.hh
+++ b/arch/x64/processor.hh
@@ -391,7 +391,9 @@ inline u64 read_xcr(u32 reg)
inline void init_fpu()
{
asm volatile ("fninit" ::: "memory");
- unsigned int csr = 0x1f80;
+ u16 cw = 0x037f;
+ asm volatile ("fldcw %0" : : "m" (cw));
+ u32 csr = 0x1f80;
asm volatile ("ldmxcsr %0" : : "m" (csr));
}
What do you think?
Waldek
PS. The diff above also changes csr type to u32 to make it consistent to
changes made by this patch
-
https://github.com/cloudius-systems/osv/commit/d5cf37af8be57beed4c78c7fa95ac135d96640e8
- adding capability for feenableexcept
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