On Mon, Apr 12, 2010 at 3:34 PM, Avi Kivity <avi@xxxxxxxxxx> wrote: > On 04/08/2010 01:51 AM, Cam Macdonell wrote: > > (sorry about the late review) > >> + >> +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. >> > > Does the value written show up in the status register? If yes, it can get > overwritten by other interrupts. If not, the lower 16 bits should be > reserved to the value 1 for future expansion. Basically it means that the > pci interrupt is equivalent to to vector 1. The status register is only 1 or 0. I've made it so 1 is the only value written to trigger an interrupt. > >> + >> +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. >> > > Suggest making this a bitfield, define bit 0 as 'at least some other machine > has signalled you' and bit 1 as 'at least one other machine 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. >> > > Come to think about it, the guest has joined is actually pointless. Since > it hasn't initialized yet you can't talk to it. So it's best to leave it > completely to the application, which can initialize shared memory and start > sending interrupts. An application defined protocol can handle joining. Good point. > How is initialization performed? I guess we can define memory to start > zeroed and let participants compete to acquire a lock. No initialization of the memory occurs presently. With interrupts the shared memory server could zero the memory. Without the server (non-interrupt case) the guests can try and open the shared memory with O_EXCL first and zero the memory if it succeeds. If O_EXCL fails, then guest would open without O_EXCL and not initialize. > > Need to document the mask register. Currently only applies with regular interrupts. Since the status register is only 0 or 1, then only the first bit has any affect. I'll add this to the spec. > > Do we want an interrupt on a guest leaving? Let's not complicate things. Probably not if we don't have one on join. Cam -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
