On 2012-10-22 16:00, Gleb Natapov wrote: > On Mon, Oct 22, 2012 at 03:25:49PM +0200, Jan Kiszka wrote: >> On 2012-10-22 15:08, Gleb Natapov wrote: >>> On Mon, Oct 22, 2012 at 03:05:58PM +0200, Jan Kiszka wrote: >>>> On 2012-10-22 14:58, Avi Kivity wrote: >>>>> On 10/22/2012 02:55 PM, Jan Kiszka wrote: >>>>>>> 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. >>>>>> >>>>>> When taking an exit for A, I'm not interesting in flushing stuff for B >>>>>> unless I have a dependency. Thus, buffers would have to be per device >>>>>> before extending their use. >>>>> >>>>> Any mmio exit has to flush everything. For example a DMA caused by an >>>>> e1000 write has to see any writes to the framebuffer, in case the guest >>>>> is transmitting its framebuffer to the outside world. >>>> >>>> We already flush when that crazy guest actually accesses the region, no >>>> need to do this unconditionally. >>>> >>> What if framebuffer is accessed from inside the kernel? Is this case handled? >> >> Unless I miss a case now, there is no direct access to the framebuffer >> possible when we are also doing coalescing. Everything needs to go >> through userspace. >> > Yes, with frame buffer is seems to be the case. One can imagine ROMD > device that is MMIO on write but still can be accessed for read from > kernel, but it cannot be coalesced even if coalesced buffer is flushed > on every exit. Usually, a ROMD device has a stable content as long as it is fast read/slow write. Once it switches mode, it is slow read as well. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux -- 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
