--- docs/specs/ivshmem_device_spec.txt | 85 ++++++++++++++++++++++++++++++++++++ 1 files changed, 85 insertions(+), 0 deletions(-) create mode 100644 docs/specs/ivshmem_device_spec.txt
diff --git a/docs/specs/ivshmem_device_spec.txt b/docs/specs/ivshmem_device_spec.txt new file mode 100644 index 0000000..9895782 --- /dev/null +++ b/docs/specs/ivshmem_device_spec.txt @@ -0,0 +1,85 @@ + +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. The IVPosition register is read-only and reports the guest's ID +number. 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 and 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. + +An interrupt is also generated when a new guest accesses the shared memory +region. A status of (2^32 - 1) indicates that a new guest has joined. + +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 1 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 can vary but it is set when the VM is started, however vector 0 is +used to notify that a new guest has joined. Guests should not use vector 0 for +any other purpose. + +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. -- 1.6.0.6 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html