Exemplo de uso da API do KVM

Provisionando uma pequena máquina virtual para executar um código que apenas imprime a mensagem deste print.

Conforme dito na seção sobre o KVM, sua API é disponibilizada através de operações ioctl() sobre o arquivo /dev/kvm. Para que VMs possam ser provisionadas, uma aplicação no espaço de usuário deverá interagir com esta API.

O código que será executado na máquina virtual é carregado na memória e informado para o KVM. Todas as operações de I/O e outras operações que geram VM Exits deverão ser tratadas pelo processo.

O que este exemplo faz é carregar um código de máquina 16-bit na memória e pedir para o KVM executá-lo. Esse código será executado em real mode e as operações de saída de cada caractere (escritas na porta serial da VM 0x3F8, também chamada de "COM1" em sistemas Microsoft) serão tratados pelo processo, imprimindo no terminal cada um destes caracteres.

Este código faz parte de um famoso artigo chamado Using the KVM API, escrito por Josh Triplett.

O código abaixo pode ser compilado e executado com:

$ gcc kvmtest.c -o kvm
$ ./kvm

/* Sample code for /dev/kvm API
 *
 * Copyright (c) 2015 Intel Corporation
 * Author: Josh Triplett <[e-mail]>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */
#include <err.h>
#include <fcntl.h>
#include <linux/kvm.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>

int main(void)
{
    int kvm, vmfd, vcpufd, ret;
    const uint8_t code[] = {
        0xba, 0xf8, 0x03,
        0xb0, 'T',
        0xee,
        0xb0, 'E',
        0xee,
        0xb0, 'S',
        0xee,
        0xb0, 'T',
        0xee,
        0xb0, 'E',
        0xee,
        0xb0, ' ',
        0xee,
        0xb0, 'D',
        0xee,
        0xb0, 'O',
        0xee,
        0xb0, ' ',
        0xee,
        0xb0, 'V',
        0xee, 
        0xb0, 'I',
        0xee,
        0xb0, 'T',
        0xee,
        0xb0, 'A',
        0xee,
        0xb0, 'L',
        0xee,
        0xb0, 'I',
        0xee,
        0xb0, 'N',
        0xee,
        0xb0, 'O',
        0xee,
        0xb0, '\n',
        0xee,
        0xb0, 'C',
        0xee,
        0xb0, 'O',
        0xee,
        0xb0, 'D',
        0xee,
        0xb0, 'I',
        0xee,
        0xb0, 'G',
        0xee,
        0xb0, 'O',
        0xee,
        0xb0, ' ',
        0xee,
        0xb0, 'E',
        0xee,
        0xb0, 'X',
        0xee,
        0xb0, 'E',
        0xee,
        0xb0, 'C',
        0xee,
        0xb0, 'U',
        0xee,
        0xb0, 'T',
        0xee,
        0xb0, 'A',
        0xee,
        0xb0, 'D',
        0xee,
        0xb0, 'O',
        0xee,
        0xb0, ' ',
        0xee,
        0xb0, 'D',
        0xee,
        0xb0, 'E',
        0xee,
        0xb0, 'N',
        0xee,
        0xb0, 'T',
        0xee,
        0xb0, 'R',
        0xee,
        0xb0, 'O',
        0xee,
        0xb0, ' ',
        0xee,
        0xb0, 'D',
        0xee,
        0xb0, 'E',
        0xee,
        0xb0, ' ',
        0xee,
        0xb0, 'V',
        0xee,
        0xb0, 'M',
        0xee,
        0xb0, ' ',
        0xee,
        0xb0, 'K',
        0xee,
        0xb0, 'V',
        0xee,
        0xb0, 'M',
        0xee,

	0xb0, '\n',       /* mov $'\n', %al */
        0xee,             /* out %al, (%dx) */
        0xf4,             /* hlt */
    };
    uint8_t *mem;
    struct kvm_sregs sregs;
    size_t mmap_size;
    struct kvm_run *run;

    kvm = open("/dev/kvm", O_RDWR | O_CLOEXEC);
    if (kvm == -1)
        err(1, "/dev/kvm");

    /* Make sure we have the stable version of the API */
    ret = ioctl(kvm, KVM_GET_API_VERSION, NULL);
    if (ret == -1)
        err(1, "KVM_GET_API_VERSION");
    if (ret != 12)
        errx(1, "KVM_GET_API_VERSION %d, expected 12", ret);

    vmfd = ioctl(kvm, KVM_CREATE_VM, (unsigned long)0);
    if (vmfd == -1)
        err(1, "KVM_CREATE_VM");

    /* Allocate one aligned page of guest memory to hold the code. */
    mem = mmap(NULL, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (!mem)
        err(1, "allocating guest memory");
    memcpy(mem, code, sizeof(code));

    /* Map it to the second page frame (to avoid the real-mode IDT at 0). */
    struct kvm_userspace_memory_region region = {
        .slot = 0,
        .guest_phys_addr = 0x1000,
        .memory_size = 0x1000,
        .userspace_addr = (uint64_t)mem,
    };
    ret = ioctl(vmfd, KVM_SET_USER_MEMORY_REGION, &region);
    if (ret == -1)
        err(1, "KVM_SET_USER_MEMORY_REGION");

    vcpufd = ioctl(vmfd, KVM_CREATE_VCPU, (unsigned long)0);
    if (vcpufd == -1)
        err(1, "KVM_CREATE_VCPU");

    /* Map the shared kvm_run structure and following data. */
    ret = ioctl(kvm, KVM_GET_VCPU_MMAP_SIZE, NULL);
    if (ret == -1)
        err(1, "KVM_GET_VCPU_MMAP_SIZE");
    mmap_size = ret;
    if (mmap_size < sizeof(*run))
        errx(1, "KVM_GET_VCPU_MMAP_SIZE unexpectedly small");
    run = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, vcpufd, 0);
    if (!run)
        err(1, "mmap vcpu");

    /* Initialize CS to point at 0, via a read-modify-write of sregs. */
    ret = ioctl(vcpufd, KVM_GET_SREGS, &sregs);
    if (ret == -1)
        err(1, "KVM_GET_SREGS");
    sregs.cs.base = 0;
    sregs.cs.selector = 0;
    ret = ioctl(vcpufd, KVM_SET_SREGS, &sregs);
    if (ret == -1)
        err(1, "KVM_SET_SREGS");

    /* Initialize registers: instruction pointer for our code, addends, and
     * initial flags required by x86 architecture. */
    struct kvm_regs regs = {
        .rip = 0x1000,
        .rax = 2,
        .rbx = 2,
        .rflags = 0x2,
    };
    ret = ioctl(vcpufd, KVM_SET_REGS, &regs);
    if (ret == -1)
        err(1, "KVM_SET_REGS");

    /* Repeatedly run code and handle VM exits. */
    while (1) {
        ret = ioctl(vcpufd, KVM_RUN, NULL);
        if (ret == -1)
            err(1, "KVM_RUN");
        switch (run->exit_reason) {
        case KVM_EXIT_HLT:
            /* puts("KVM_EXIT_HLT"); */
            return 0;
        case KVM_EXIT_IO:
            if (run->io.direction == KVM_EXIT_IO_OUT && run->io.size == 1 && run->io.port == 0x3f8 && run->io.count == 1)
                putchar(*(((char *)run) + run->io.data_offset));
            else
                errx(1, "unhandled KVM_EXIT_IO");
            break;
        case KVM_EXIT_FAIL_ENTRY:
            errx(1, "KVM_EXIT_FAIL_ENTRY: hardware_entry_failure_reason = 0x%llx",
                 (unsigned long long)run->fail_entry.hardware_entry_failure_reason);
        case KVM_EXIT_INTERNAL_ERROR:
            errx(1, "KVM_EXIT_INTERNAL_ERROR: suberror = 0x%x", run->internal.suberror);
        default:
            errx(1, "exit_reason = 0x%x", run->exit_reason);
        }
    }
}