On Wed, Mar 30, 2011 at 12:50:53PM +0200, Avi Kivity wrote: > On 03/30/2011 12:47 PM, Gleb Natapov wrote: > >On Wed, Mar 30, 2011 at 12:38:53PM +0200, Avi Kivity wrote: > >> On 03/29/2011 02:08 PM, Gleb Natapov wrote: > >> >Currently we sync registers back and forth before/after exiting > >> >to userspace for IO, but during IO device model shouldn't need to > >> >read/write the registers, so we can as well skip those sync points. The > >> >only exaception is broken vmware backdor interface. The new code sync > >> >registers content during IO only if registers are read from/written to > >> >by userspace in the middle of the IO operation and this almost never > >> >happens in practise. > >> > >> While this is a nice idea, how much does it save in practice? It > >> does introduce more complexity. > >> > > > >I haven't measured, but can try to do so. It eliminates two copies of > >all registers on each MMIO/PIO read. I expect this to be measurable in > >workloads that do many such reads. > > > > I don't, especially if these are mmios to userspace. Perhaps it's > better to remove the copy on kernel mmio, since it's much faster, if > the result is simpler (there can be no KVM_SET_REGS in that > context). > The patch saves copying of 256 bytes on each MMIO/PIO read. It may not save a lot comparing to time it takes to do one MMIO to userspace, but do 1 million of those and you saved a lot of CPU cycles. I do not think we should abandon the optimization so easily. Unfortunately I can't run perf on 2.6.38 kernel right now. It gives me strange errors and when it doesn't it makes kernel OOPS. -- 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
