+++ /dev/null
-
-Device Specification for Inter-VM shared memory device
-------------------------------------------------------
-
-The Inter-VM shared memory device is designed to share a region of memory to
-userspace in multiple virtual guests. The memory region does not belong to any
-guest, but is a POSIX memory object on the host. Optionally, the device may
-support sending interrupts to other guests sharing the same memory region.
-
-
-The Inter-VM PCI device
------------------------
-
-*BARs*
-
-The device supports three BARs. BAR0 is a 1 Kbyte MMIO region to support
-registers. BAR1 is used for MSI-X when it is enabled in the device. BAR2 is
-used to map the shared memory object from the host. The size of BAR2 is
-specified when the guest is started and must be a power of 2 in size.
-
-*Registers*
-
-The device currently supports 4 registers of 32-bits each. Registers
-are used for synchronization between guests sharing the same memory object when
-interrupts are supported (this requires using the shared memory server).
-
-The server assigns each VM an ID number and sends this ID number to the QEMU
-process when the guest starts.
-
-enum ivshmem_registers {
- IntrMask = 0,
- IntrStatus = 4,
- IVPosition = 8,
- Doorbell = 12
-};
-
-The first two registers are the interrupt mask and status registers. Mask and
-status are only used with pin-based interrupts. They are unused with MSI
-interrupts.
-
-Status Register: The status register is set to 1 when an interrupt occurs.
-
-Mask Register: The mask register is bitwise ANDed with the interrupt status
-and the result will raise an interrupt if it is non-zero. However, since 1 is
-the only value the status will be set to, it is only the first bit of the mask
-that has any effect. Therefore interrupts can be masked by setting the first
-bit to 0 and unmasked by setting the first bit to 1.
-
-IVPosition Register: The IVPosition register is read-only and reports the
-guest's ID number. The guest IDs are non-negative integers. When using the
-server, since the server is a separate process, the VM ID will only be set when
-the device is ready (shared memory is received from the server and accessible via
-the device). If the device is not ready, the IVPosition will return -1.
-Applications should ensure that they have a valid VM ID before accessing the
-shared memory.
-
-Doorbell Register: To interrupt another guest, a guest must write to the
-Doorbell register. The doorbell register is 32-bits, logically divided into
-two 16-bit fields. The high 16-bits are the guest ID to interrupt and the low
-16-bits are the interrupt vector to trigger. The semantics of the value
-written to the doorbell depends on whether the device is using MSI or a regular
-pin-based interrupt. In short, MSI uses vectors while regular interrupts set the
-status register.
-
-Regular Interrupts
-
-If regular interrupts are used (due to either a guest not supporting MSI or the
-user specifying not to use them on startup) then the value written to the lower
-16-bits of the Doorbell register results is arbitrary and will trigger an
-interrupt in the destination guest.
-
-Message Signalled Interrupts
-
-A ivshmem device may support multiple MSI vectors. If so, the lower 16-bits
-written to the Doorbell register must be between 0 and the maximum number of
-vectors the guest supports. The lower 16 bits written to the doorbell is the
-MSI vector that will be raised in the destination guest. The number of MSI
-vectors is configurable but it is set when the VM is started.
-
-The important thing to remember with MSI is that it is only a signal, no status
-is set (since MSI interrupts are not shared). All information other than the
-interrupt itself should be communicated via the shared memory region. Devices
-supporting multiple MSI vectors can use different vectors to indicate different
-events have occurred. The semantics of interrupt vectors are left to the
-user's discretion.
-
-
-Usage in the Guest
-------------------
-
-The shared memory device is intended to be used with the provided UIO driver.
-Very little configuration is needed. The guest should map BAR0 to access the
-registers (an array of 32-bit ints allows simple writing) and map BAR2 to
-access the shared memory region itself. The size of the shared memory region
-is specified when the guest (or shared memory server) is started. A guest may
-map the whole shared memory region or only part of it.
Code Review / kvmfornfv.git / blobdiff