VMX(3)                                                     VMX(3)

          vmx - x86 virtualization interface


          The vmx device supports "virtual CPUs" using the Intel VT-x
          extension (a.k.a. VMX instruction set).  This is used by
          vmx(3) to implement virtual machines.  Access to the vmx
          device is restricted to the hostowner.

          The top level directory contains a clone file and numbered
          subdirectories representing the allocated virtual CPUs.
          Opening the clone file allocates a new virtual CPU and
          returns the file descriptor to its ctl file.  The ctl file
          provides the main control interface. See below for a list of
          commands. Reading returns the subdirectory number.  Removing
          the ctl file marks the virtual CPU as moribund.  The status
          file contains the current status of the virtual CPU, which
          is one of

          init     The virtual CPU is being initialized.
          ready    The virtual CPU is idle.
          running  The virtual CPU is executing code.
          dead     The virtual CPU suffered a fatal error.  This state
                   may be followed by an error message.
          ending   The virtual CPU is shutting down.

          The map file contains the memory map that the virtual CPU
          will see.  It consists of lines of the form
               access cache lowaddr highaddr segment offset
          Lowaddr specifies the lowest address in the region and high-
          addr one past the highest address.  The region is mapped to
          a region of the same size in the global segment segment (see
          segment(3)), starting at offset.  The access field specifies
          the permitted types of access using the characters r (read),
          w (write), x (execute) and - (padding character).  The cache
          field specifies the cacheability of the region, it must be
          one of uc, wc, wt, wp and wb (as defined in the Intel SDM).

          Writes to the map file append lines to the end.  Multiple

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     VMX(3)                                                     VMX(3)

          lines can be written at once but all lines written must be
          newline terminated.  Regions can be overlapping, in which
          case later definitions always override earlier ones.  The
          map can be cleared by opening the file with OTRUNC (see open

          The regs file contains the registers of the virtual CPU in
          the format name value.  Writes to the file (in the same for-
          mat) write to the referenced registers (if possible).  Mul-
          tiple lines can be written at once but all lines written
          must be newline terminated.

          Some registers (CR0 and CR4) are split into three registers,
          suffixed real, fake and mask.  In this case, real corre-
          sponds to the bits that affect actual CPU execution, fake
          corresponds to the bits read back by the guest and the bits
          set in mask are those "owned" by the host.  The guest is
          free to modify the bits that it owns (in which case it
          always has the same value in both real and fake), but
          attempting to change a host-owned bit from the status in
          fake causes a VM exit.  Certain bits are owned by the ker-
          nel, which means they are fixed in both mask and real.

          Reading the wait file will stall the reading process until
          the virtual CPU reaches a point where it cannot continue (a
          "VM exit").  This may be due to the an access to hardware or
          a software exception.  Each exit is indicated by a single
          line in a format compatible with tokenize (see
          getfields(2)). The first column contains the cause of the
          exit and the second column contains the "exit qualification"
          field that may contain more details on the exit (see Intel
          SDM).  The remaining columns come in pairs and contain fur-
          ther info and the values of relevant registers.

          Some notable exit causes are (see kernel source code for a
          complete list)

          #exception   Exception of the specified type (e.g. #gp for
                       general protection fault).  Currently only
                       debug exceptions are configured to cause VM
          triplef      Triple fault.
          eptfault     The virtual CPU attempted a memory access that
                       does not match any entry in the map file.
          movcr        Illegal access to a control register (see
          .instr       The virtual CPU attempted to execute the
                       instruction instr.

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     VMX(3)                                                     VMX(3)

          *ack         Not an actual exit, but acknowledgement that an
                       interrupt request (IRQ) was posted.

          The fpregs file contains the virtual CPU's floating point
          registers, in the same binary format used by proc(3).

        Control messages
          quit              Destroy the current virtual CPU.
          go [ regs ]       Launch the virtual CPU.  Regs is an
                            optional list of register changes in the
                            format name=value; that will be applied
                            before launching.
          stop              Stop the virtual CPU.
          step [ regs ]     Executes a single instruction with the
                            virtual CPU.  Regs is optinal, same as go.
          exc excep         The exception excep is triggered in the
                            virtual CPU.  Excep can either be a named
                            exception (such as #gp, in lower case) or
                            an exception number.  A number may be
                            preeded by # to mark it as an exception,
                            otherwise it is delivered as an interrupt
                            (but always disregarding whether inter-
                            rupts are enabled).
          irq [ excep ]     An Interrupt is posted, i.e. the exception
                            excep will be triggered the next time
                            interrupts are enabled in the virtual CPU,
                            at which point a *ack message is sent to
                            wait.  Irq cancels any interrupts that
                            have been previously posted but not yet
                            delivered and it can be called with no
                            argument to cancel an interrupt.
          extrap bitmap     Changes the exception bitmap. Set bits
                            cause a VM exits.


          vmx(1), cpuid(8)

          Intel 64 and IA-32 Architectures Software Developer's Man-
          ual, Volume 3B, Chapters 23-33.

          Devvmx can and will crash your kernel.

          Devvmx first appeared in 9front (June, 2017).

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