On 2011-09-30 10:18, David Gibson wrote: > With PowerKVM, exits from KVM to qemu are even more expensive than on > x86. One significant source of these we're finding (since we usually > work in -nographic mode) is the nographic_timer. > > At present, we're using a hack to disable it, but that's obviously not > a long term solution. From examination, it looks like the only > purpose of this timer is to flush coalesced mmios. So it seems like > the timer should only be activated when a coalesced mmio region > actually exists, but I'm not entirely sure how to go about that. > > Thinking longer term, it seems very odd that a userspace periodic > timer handles this at all. Surely it would make more sense to use a > kernel timer within KVM, which can be activated only when there are > actually pending coalesced MMIOs in the buffer.
Coalesced MMIO should only be flushed when a device depending on it gets accessed - either by a VCPU or by the iothread (to update the graphic output e.g.). We are working on such a concept (to reduce latency for VCPUs with real-time constraints). And long-term, we need per-device MMIO buffers to avoid flushing unrelated content. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
