On Tue, Feb 09, 2016 at 11:46:08AM +0100, Igor Mammedov wrote: > > > > > 2. ACPI approach consumes guest usable RAM to allocate buffer > > > > > and then makes device to DMA data in that RAM. > > > > > That's a design point I don't agree with. > > > > > > > > Blame the broken VM GEN ID spec. > > > > > > > > For VM GEN ID, instead of DMA we could make ACPI code read PCI BAR and > > > > copy data over, this would fix rebalancing but there is a problem with > > > > this approach: it can not be done atomically (while VM is not yet > > > > running and accessing RAM). So you can have guest read a partially > > > > corrupted ID from memory. > > > > > > Yep, VM GEN ID spec is broken and we can't do anything about it as > > > it's absolutely impossible to guaranty atomic update as guest OS > > > has address of the buffer and can read it any time. Nothing could be > > > done here. > > > > Hmm I thought we can stop VM while we are changing the ID. > > But of course VM could be accessing it at the same time. > > So I take this back, ACPI code reading PCI BAR and > > writing data out to the buffer would be fine > > from this point of view. > spec is broken regardless of who reads BAR or RAM as read > isn't atomic and UUID could be updated in the middle. > So MS will have to live with that, unless they have a secret > way to tell guest stop reading from that ADDR, do update and > signal guest that it can read it.
And really, that the issue that's driving this up to v19. There's no good way to implement a bad spec. It feels more like a failed experiment than like a thought through interface. So maybe we should leave this alone, wait until we see an actual user - this way we can figure out the implementation constraints better. -- MST
