[Qemu-devel] Re: [PATCH v4 1/3] Device specification for shared memory PCI device

[Avi Kivity]

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.

> +
> +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.

How is initialization performed?  I guess we can define memory to start 
zeroed and let participants compete to acquire a lock.

Need to document the mask register.

Do we want an interrupt on a guest leaving?  Let's not complicate things.


--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.