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. 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
