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The only true difference is that derive_clock_uncertainty will derive different uncertainties between a virtual/external clock and internal clock. Right now it derives uncertainty to the same clock, which will be less. It's probably going to be a 10ps difference, but true.
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But if you use the incoming clock to e.g. clock the data into a fifo, it is the same clock (externally and internally). If the difference of 10 ps is valid, shouldn't the 'derive_clock_uncertainty' be fixed?
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I like the virtual clock for multiple reasons. You can use it as a tag for report_timing, i.e.:
report_timing -setup/-hold -from ext_clk...
Will report timing on all of these input paths, without having to describe the ports. It's not a big deal here since the inputs are a bus, but on interfaces with 20 different port names, it helps.
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But wouldn't "report_timing -setup/-hold -from fga_clk" work as well?
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