Obsolete

This Readme is obsolete due to a new kernel version.

Installation

Copy this files into your linux kernel.

For new kernel:

vmx.c is going to /arch/x86/kvm/vmx
svm.c is going to /arch/x86/kvm/svm

Don't forget to disable rdtscp in your qemu xml config:

Add rdtscp=off to your qemu:arg, so it will look similar to this:

Changing timer

You can play with ticks if you want to.

For Intel users:

Open vmx.c in text editor
Find handle_rdtsc function
Change u64 fake_diff = diff / 16;
16 is a divider of actual difference in timestamp, you can increase and decrease it

For AMD users:

Open svm.c in text editor
Find handle_rdtsc_interception function
Change u64 fake_diff = diff / 20;
20 is a divider of actual difference in timestamp, you can increase and decrease it

Modifying kernel by hand

If you have trouble to build your kernel with files from this repository, you can try to modify vmx and svm files in your kernel by yourself.

Changes for Intel users

Open vmx.c in text editor.

First you need to find _setup_vmcsconfig function (about 2500 lines) and add this line:

    CPU_BASED_CR3_LOAD_EXITING |
    CPU_BASED_CR3_STORE_EXITING |
    CPU_BASED_UNCOND_IO_EXITING |
    CPU_BASED_MOV_DR_EXITING |
    CPU_BASED_USE_TSC_OFFSETTING |
    CPU_BASED_MWAIT_EXITING |
    CPU_BASED_MONITOR_EXITING |
    CPU_BASED_INVLPG_EXITING |
    CPU_BASED_RDPMC_EXITING |   
    CPU_BASED_RDTSC_EXITING; //added line

Next you need to find an array of pointers to handlers initialization called _static int (kvm_vmx_exit_handlers[])(struct kvm_vcpu vcpu)_ and add the line to the end:

        [EXIT_REASON_PREEMPTION_TIMER]  = handle_preemption_timer,
    [EXIT_REASON_ENCLS]     = handle_encls,
    [EXIT_REASON_BUS_LOCK]      = handle_bus_lock_vmexit,
        [EXIT_REASON_RDTSC]     = handle_rdtsc, //added line
};

Then add this code before _static int (kvm_vmx_exit_handlers[])(struct kvm_vcpu vcpu)_:

static u32 print_once = 1;

static int handle_rdtsc(struct kvm_vcpu *vcpu) 
{ 
    static u64 rdtsc_fake = 0;
    static u64 rdtsc_prev = 0;
    u64 rdtsc_real = rdtsc();

    if(print_once)
    {
        printk("[handle_rdtsc] fake rdtsc vmx function is working\n");
        print_once = 0;
        rdtsc_fake = rdtsc_real;
    }

    if(rdtsc_prev != 0)
    {
        if(rdtsc_real > rdtsc_prev)
        {
            u64 diff = rdtsc_real - rdtsc_prev;
            u64 fake_diff =  diff / 16; // if you have 4.2Ghz on your vm, change 16 to 20 
            rdtsc_fake += fake_diff;
        }
    }
    if(rdtsc_fake > rdtsc_real)
    {
        rdtsc_fake = rdtsc_real;
    }
    rdtsc_prev = rdtsc_real;
        vcpu->arch.regs[VCPU_REGS_RAX] = rdtsc_fake & -1u;
        vcpu->arch.regs[VCPU_REGS_RDX] = (rdtsc_fake >> 32) & -1u;  

        return skip_emulated_instruction(vcpu);
}

Changes for AMD users

Open svm.c in text editor.

Find _static int (const svm_exit_handlers[])(struct kvm_vcpu vcpu)_ (~2700 lines) and add this line:

    [SVM_EXIT_AVIC_INCOMPLETE_IPI]      = avic_incomplete_ipi_interception,
    [SVM_EXIT_AVIC_UNACCELERATED_ACCESS]    = avic_unaccelerated_access_interception,
    [SVM_EXIT_VMGEXIT]          = sev_handle_vmgexit,
    [SVM_EXIT_RDTSC]            = handle_rdtsc_interception, //added line
};

Next find _static void init_vmcb(struct kvmvcpu *vcpu) (~1000 lines) and add this line:

    svm_set_intercept(svm, INTERCEPT_RDPRU);
    svm_set_intercept(svm, INTERCEPT_RSM);
    svm_set_intercept(svm, INTERCEPT_RDTSC); //added line

After that add this code on top of _static int (const svm_exit_handlers[])(struct kvm_vcpu vcpu)_:

static u32 print_once = 1;

static int handle_rdtsc_interception(struct kvm_vcpu *vcpu) 
{
        static u64 rdtsc_fake = 0;
    static u64 rdtsc_prev = 0;
    u64 rdtsc_real = rdtsc();

    if(print_once)
    {
        printk("[handle_rdtsc] fake rdtsc svm function is working\n");
        print_once = 0;
        rdtsc_fake = rdtsc_real;
    }

    if(rdtsc_prev != 0)
    {
        if(rdtsc_real > rdtsc_prev)
        {
            u64 diff = rdtsc_real - rdtsc_prev;
            u64 fake_diff =  diff / 20; // if you have 3.2Ghz on your vm, change 20 to 16
            rdtsc_fake += fake_diff;
        }
    }
    if(rdtsc_fake > rdtsc_real)
    {
        rdtsc_fake = rdtsc_real;
    }
    rdtsc_prev = rdtsc_real;

    vcpu->arch.regs[VCPU_REGS_RAX] = rdtsc_fake & -1u;
        vcpu->arch.regs[VCPU_REGS_RDX] = (rdtsc_fake >> 32) & -1u;

        return skip_emulated_instruction(vcpu);
}

Kernel building

Clone the kernel from official repository or from repository of your os.
Make changes for your cpu

Install the requirements for kernel building

sudo apt-get install build-essential libncurses-dev bison flex libssl-dev libelf-dev

Copy your current config into your kernel folder
```
cp -v /boot/config-$(uname -r) .config
```
Make kernel
```
make -j $(nproc)
```

Install kernel

sudo make INSTALL_MOD_STRIP=1 modules_install && sudo make install

Reboot your system
Check if a new kernel was installed
```
uname -a
```

WCharacter / RDTSC-KVM-Handler