shitty_timer

External clock thingy to bypass timing checks (Cross platform POC only ATM)

Right now its 99.99987% accurate for 10 calls, at 10ms, and counting at 1000MHz, which means by the time 1 second passes it would be 99.99857% accurate (each second is minus 0.0013% accuracy), so it drifts a lot)

build_command

gcc -coverage -O0 ./Timer.c -o Timer

how_to_use

./Timer | number_of_tests | delay_in_ms | guest_mhz

./Timer 10 10 1000

example_output

PS C:\Timer> ./Timer 10 10 1000
==============================================
[*] RDTSC Compare Total Diff: 7466
[*] RDTSC Locky Tricky AVG Diff: 16
[*] RDTSC VMExit AVG Diff: 1635
[*] Host CPU MHz: 2594
[*] Starting Timer
[*] Timer Delay in miliseconds: 10
==============================================
Counting To: 10
Locky Tricky Timer Diff AVG: 28513285
Locky Tricky Timer AVG Diff In Seconds: 0.010992
VMExit Timer Diff AVG: 28510335
VMExit Timer AVG Diff In Seconds: 0.010991
==============================================
Timing Deviation in Ticks: -2950
Timing Deviation in Seconds: -0.000001
==============================================
Timing Deviation in Ticks: 2950
Timing Deviation in Seconds: 0.000001
==============================================
Counting up at Guest MHz: 1000
Counting up Host: 0
Counting up Guest: 0.000000
Host Clock: 103034220953184
Guest Clock: 103034220953138.170000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2250489440 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 1
Counting up Guest: 0.385682
Host Clock: 103034226713160
Guest Clock: 103034220953144.340000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2256249416 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 2
Counting up Guest: 0.771363
Host Clock: 103034230226274
Guest Clock: 103034220953150.520000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2259762530 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 3
Counting up Guest: 1.157045
Host Clock: 103034234683608
Guest Clock: 103034220953156.690000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2264219864 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 4
Counting up Guest: 1.542727
Host Clock: 103034238287260
Guest Clock: 103034220953162.860000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2267823516 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 5
Counting up Guest: 1.928409
Host Clock: 103034241989142
Guest Clock: 103034220953169.030000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2271525398 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 6
Counting up Guest: 2.314090
Host Clock: 103034251401040
Guest Clock: 103034220953175.200000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2280937296 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 7
Counting up Guest: 2.699772
Host Clock: 103034256091142
Guest Clock: 103034220953181.370000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2285627398 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 8
Counting up Guest: 3.085454
Host Clock: 103034260802568
Guest Clock: 103034220953187.550000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2290338824 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------
Counting up Host: 9
Counting up Guest: 3.471136
Host Clock: 103034264501857
Guest Clock: 103034220953193.720000
Time Scale: 0.385682
Time Multiplier: 6.170908
Host Freq: 2592.811378
Guest Freq: 1000.000000
Host CPU Registers: EAX=2294038113 | EDX=23989
Leaf Check EAX=1 and EDX=2: Leaf=8589934593
Leaf Check EAX=1 and EDX=2: Math Check=4294967298.000000
------------------------

plans_and_implementation

• (Record the time it takes on VM and real HW for a normal RDTSC call)

  (Then record it again with a forced exit call)

• Create an external Timer in the VM_EXIT section of QEMU that starts an external clock on VM boot

  • Copy the VM clock value on the first exit call

  (then catch-up to current VM RDTSC clock time) (Record how many subsequent calls are made after the first call and store it to memory)

  • Copy host clock value

  (Save value for validation later)

  (Minus largest clock value and smallest clock value and save the diff as well)

• Start clock with the VM recorded value in the previous step

• Count up normally but every VM_EXIT add to it using a random integer assigned from the fastest and slowest call in our previously recorded calls

  (if the fastest call is 10 somethings, and the slowest is 100 somethings, use a random number between them [aka add 45 somethings to our timer that is currently at 1000, so it would be 1045 now])

• Check the VM RDTSC value for accuracy every few calls (make sure it counts up at a similar rate to the host)

  (Compare VM clock to our timer (adjust our timer to fit within margin of error))

  • (Margin of error is the slowest recorded call minus our fastest call so 100ns-10ns=90ns)

    • (In this example we would adjust our timer so that it is +/-90 nanoseconds from the VM diff)

  • (Copy the timer and VM RDTSC value at their current state)

    • (Compare both and make a diff to compare with the original diff saved)

  (Compare this value to the number of recorded of calls made (and the diff of our host RDTSC call) to make sure the number is not wildly inaccurate)

• Then just passthrough the external timers value for every RDTSC call

(This is a simple overview and the code is just a bit more complex)