FEXCore: Dynamically scale TSC #4033
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When I implemented TSC scaling originally, I chose a scale factor of 128 because it basically covered the range of devices we cared about without going too high. I also only tested devices that had a TSC scale factor from 19.2Mhz to 34Mhz. Turns out there is hardware that also has a 48Mhz cycle counter, which cause them to effectively have a 6.1Ghz cycle counter, which is kind of absurd. Instead of a fixed scale, just calculate the amount of scaling we need to get >= the minimum threshold of 1Ghz. This will change the shift from 7 to 5 or 6 for the faster cycle counter devices. Of course if someone wants to know the scale factor they can still use cpuid function 15h to know it. Fixes FEX-Emu#4026
These will differ depending on which runner they are running on. Just disable them but keep the instructions around so show how bad rdtscp is, but how good rdtsc is.
alyssarosenzweig
approved these changes
Sep 3, 2024
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When I implemented TSC scaling originally, I chose a scale factor of 128 because it basically covered the range of devices we cared about without going too high. I also only tested devices that had a TSC scale factor from 19.2Mhz to 34Mhz. Turns out there is hardware that also has a 48Mhz cycle counter, which cause them to effectively have a 6.1Ghz cycle counter, which is kind of absurd.
Instead of a fixed scale, just calculate the amount of scaling we need to get >= the minimum threshold of 1Ghz. This will change the shift from 7 to 5 or 6 for the faster cycle counter devices.
Of course if someone wants to know the scale factor they can still use cpuid function 15h to know it.
Fixes #4026