Re: [Qemu-devel] [PATCH 3/4] virtio-gpu: v0.3 of the virtio based GPU code.

[Paolo Bonzini <pbonzini@redhat.com>]

Il 12/03/2014 21:26, Michael S. Tsirkin ha scritto:
>> +Event queue:
>> +The only current event passed is a message to denote the host
>> +wants to update the layout of the screens. It contains the same
>> +info as the response to VIRTGPU_CMD_GET_DISPLAY_INFO.

I wonder if an event queue is the best mechanism if you can get the same 
info from a command anyway.  For virtio-scsi I used a queue because I 
needed to specify which target or LUN the event applied to, but here you 
do not need it and a queue is susceptible to dropped events.

Perhaps a configuration field is better, like this:

     u32 events_read;
     u32 events_clear;

A new event sets a bit in events_read and generates a configuration 
change interrupt.  The guest should never write to events_read.

Writing to events_clear has the side effect of the device doing 
"events_read &= ~events_clear".  We cannot have R/W1C fields in virtio, 
but this approximation is good enough.

When the guest receives a configuration change interrupt, it reads 
event_read.  If it is nonzero, it writes the same value it read to 
events_clear, and sends the necessary commands to the card in order to 
retrieve the event data.  It can then read again event_read, and loop if 
it is again nonzero.

Paolo