Am 02.07.2012 12:18, schrieb Peter Crosthwaite: > On Mon, Jul 2, 2012 at 8:01 PM, Kevin Wolf <kw...@redhat.com> wrote: >> Am 02.07.2012 11:42, schrieb Peter Crosthwaite: >>> On Mon, Jul 2, 2012 at 7:04 PM, Kevin Wolf <kw...@redhat.com> wrote: >>>> Am 02.07.2012 10:57, schrieb Peter Crosthwaite: >>>>> No conditional on the qemu_coroutine_create. So it will always create >>>>> a new coroutine for its work which will solve my problem. All I need >>>>> to do is pump events once at the end of machine model creation. Any my >>>>> coroutines will never yield or get queued by block/AIO. Sound like a >>>>> solution? >>>> >>>> If you don't need the read data in your initialisation code, >>> >>> definately not :) Just as long as the read data is there by the time >>> the machine goes live. Whats the current policy with bdrv_read()ing >>> from init functions anyway? Several devices in qemu have init >>> functions that read the entire storage into a buffer (then the guest >>> just talks to the buffer rather than the backing store). >> >> Reading from block devices during device initialisation breaks >> migration, so I'd like to see it go away wherever possible. Reading in >> the whole image file doesn't sound like something for which a good >> excuse exists, > > Makes sense for small devices on embedded platforms. You end up with a > very simple and clean device model. The approach is totally broken for > HDDs but it does make some sense for the little embedded flashes where > you can get away with caching the entire device storage in RAM for the > lifetime of the system.
It kind of works for read-only devices, it's just ugly there. With writeable devices it makes the VM unmigratable. >> you can do that as well during the first access. >> > > Kind of painful though to change this for a lot of existing devices. > Its a reasonable policy for new devices, but can we just fix the > init->bdrv_read() calls in place for the moment? Sure, but you asked for the policy and the policy is "only with good reasons". ;-) >>> Pflash (pflash_cfi_01.c) is the device that is causing me interference >>> here and it works exactly like this. If we make the bdrv_read() aio >>> though, how do you ensure that it has completed before the guest talks >>> to the device? Will this just happen at the end of machine_init >>> anyways? Can we put a one liner in the machine init framework that >>> pumps all AIO events then just mass covert all these bdrv_reads (in >>> init functions) to bdrv_aio_read with a nop completion callback? >> >> The initialisation function of the device can wait at its end for all >> AIOs to return. > > You lose the performance increase discussed below however, as instead > of the init function returning to continue on with the machine init, > you block on disk IO. Do you think it really matters? I guess it might if you have two such devices that take each a second or so to load, but otherwise there isn't much to parallelise. Kevin