gem5 has been extensively used in computer architecture simulations and in the evaluation of new architectures for HPC (high performance computing) systems. Previous work has validated gem5 against ARM platforms. However, gem5 still shows high inaccuracy when modeling Intel's x86 based processors. In this work, we focus on the simulation of a single node high performance system and study the sources of inaccuracies of gem5. Then we validate gem5 simulator against an Intel processor, Core-i7 (Haswell microarchitecture). We configured gem5 as close as possible to match Core-i7 Haswell microarchitecture configurations and made changes to the simulator to add some features, modified existing code, and tuned built-in configurations. As a result, we validated the simulator by fixing many sources of errors to match real hardware results with less than 6% mean error rate for different control, memory, dependency and execution microbenchmarks.