On Mon, Oct 22, 2012 at 02:55:24PM +0200, Avi Kivity wrote: > On 10/22/2012 02:53 PM, Gleb Natapov wrote: > > On Mon, Oct 22, 2012 at 02:45:37PM +0200, Jan Kiszka wrote: > >> On 2012-10-22 14:18, Avi Kivity wrote: > >> > On 10/22/2012 01:45 PM, Jan Kiszka wrote: > >> > > >> >>> Indeed. git pull, recheck and call for kvm_flush_coalesced_mmio_buffer() > >> >>> is gone. So this will break new userspace, not old. By global you mean > >> >>> shared between devices (or memory regions)? > >> >> > >> >> Yes. We only have a single ring per VM, so we cannot flush multi-second > >> >> VGA access separately from other devices. In theory solvable by > >> >> introducing per-region rings that can be driven separately. > >> > > >> > But in practice unneeded. Real time VMs can disable coalescing and not > >> > use planar VGA modes. > >> > >> A) At least right now, we do not differentiate between the VGA modes and > >> if flushing is needed. So that device is generally taboo for RT cores of > >> the VM. > >> B) We need to disable coalescing in E1000 as well - if we want to use > >> that model. > >> C) Gleb seems to propose using coalescing far beyond those two use cases. > >> > > Since the userspace change is needed the idea is dead, but if we could > > implement it I do not see how it can hurt the latency if it would be the > > only mechanism to use coalesced mmio buffer. Checking that the ring buffer > > is empty is cheap and if it is not empty it means that kernel just saved > > you a lot of 8 bytes exists so even after iterating over all the entries there > > you still saved a lot of time. > > It's time where the guest cannot take interrupts, and time in a high > priority guest thread that is spent processing low guest priority requests. > Proposed fix has exactly same issue. Until all data is transfered to userspace no interrupt will be served. -- Gleb. -- 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
