Achilleas Anagnostopoulos fd862d0d15 rt0_64: preserve XMM registers when calling gate handlers ...

Some gate handlers may invoke Go runtime code that clobbers the XMM
registers that could be in use by the code that triggered the gate
handler.

One case where this happens is when a growing a slice and the new
location for the slice data resides in a RO page with a CoW flag.  In
this scenario, runtime.growslice will invoke runtime.memmove which may
take a code path that uses XMM registers for copying data around.

During the copy, a page fault occurs and the kernel page fault handler
will detect the CoW flag, allocate a new frame and copy the original
data to the new frame before resuming execution. As the page copy is
performed using the built-in copy function this will cause the XMM
registers to be clobbered.

To prevent this from happening, the asm gate code that gets executed
when an exception occurs will now preserve the XMM regs on the stack
before invoking the registered exception handler.

In addition, for Go 1.9+ the gate code will also temporarily disable use
of AVX instructions by runtime.memmove by setting runtime.useAVXmemmove
to 0. This is required so the gate does not need to also preserve any
AVX registers.

2018-04-28 11:11:57 +01:00

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