Impact: Kernel & all software
Mitigation: recompile software and kernel with a modified compiler that introduces the LFENCE opcode at the proper positions in the resulting code
Performance impact of the mitigation: negligible

CVE-2017-5715: branch target injection (Spectre Variant 2)

Impact: Kernel
Mitigation 1: new opcode via microcode update that should be used by up to date compilers to protect the BTB (by flushing indirect branch predictors)
Mitigation 2: introducing "retpoline" into compilers, and recompile software/OS with it
Performance impact of the mitigation: high for mitigation 1, medium for mitigation 2, depending on your CPU

CVE-2017-5754: rogue data cache load (Meltdown)

Impact: Kernel
Mitigation: updated kernel (with PTI/KPTI patches), updating the kernel is enough
Performance impact of the mitigation: low to medium

Example of the output of the script:

$ sudo ./spectre-meltdown-checker.sh
Spectre and Meltdown mitigation detection tool v0.07

CVE-2017-5753 [bounds check bypass] aka 'Spectre Variant 1'
* Kernel compiled with LFENCE opcode inserted at the proper places:  NO  (only 38 opcodes found, should be >= 60)
> STATUS:  VULNERABLE

CVE-2017-5715 [branch target injection] aka 'Spectre Variant 2'
* Mitigation 1
*   Hardware (CPU microcode) support for mitigation:  NO
*   Kernel support for IBRS:  NO
*   IBRS enabled for Kernel space:  NO
*   IBRS enabled for User space:  NO
* Mitigation 2
*   Kernel compiled with retpolines:  NO
> STATUS:  VULNERABLE  (IBRS hardware + kernel support OR kernel with retpolines are needed to mitigate the vulnerability)

CVE-2017-5754 [rogue data cache load] aka 'Meltdown' aka 'Variant 3'
* Kernel supports Page Table Isolation (PTI):  YES
* PTI enabled and active:  YES
> STATUS:  NOT VULNERABLE  (PTI mitigates the vulnerability)