On Wed, Jul 13, 2022 at 11:43:20AM +0530, Arnabjyoti Kalita wrote: > Dear all, > > How do I know if TSC scaling is being done when a VM is running in KVM > mode? If TSC scaling is being done, how do I calculate the multiplier > that needs to be added to host TSC to get guest TSC? You can check vmx_write_tsc_offset/vmx_write_tsc_multiplier, and then follow Intel SDM to calculate guest TSC: (Guest) RDTSC first computes the product of the value of the (Host) IA32_TIME_STAMP_COUNTER MSR and the value of the TSC multiplier. It then shifts the value of the product right 48 bits and loads EAX:EDX with the sum of that shifted value and the value of the TSC offset. > > For more context, I am running IntelPT and collecting Intel PT traces > when a VM Is running in KVM mode and I see that the TSC values differ > by a multiplier when the VM is running in root mode (between a vmexit > and next vmentry) vs non-root mode (between a vmentry and next > vmexit). I use QEMU - 5.2.0 to run a guest and I use an Intel > Kaby-Lake microarchitecture. > > For e.g. The first TSC packet that I get from Intel PT when guest is > in root mode is 0xb24d498c651 > while the first TSC packet that I get from Intel PT when guest is in > non-root mode is 0x4b41048e77. TSC value in Intel PT packet in non-root mode follows the same calculation above. Chao > > Best Regards, > Arnabjyoti Kalita
