On 10/07/2024 10:52, Ilias Stamatis wrote:
There is no direct way for userspace to be notified about coalesced MMIO writes when using KVM_REGISTER_COALESCED_MMIO. If the next MMIO exit is when the ring buffer has filled then a substantial (and unbounded) amount of time may have passed since the first coalesced MMIO. To improve this, make it possible for userspace to use poll() and select() on the fd returned by the KVM_CREATE_COALESCED_MMIO_BUFFER ioctl. This way a userspace VMM could have dedicated threads that deal with writes to specific MMIO zones. For example, a common use of MMIO, particularly in the realm of network devices, is as a doorbell. A write to a doorbell register will trigger the device to initiate a DMA transfer. When a network device is emulated by userspace a write to a doorbell register would typically result in an MMIO exit so that userspace can emulate the DMA transfer in a timely manner. No further processing can be done until userspace performs the necessary emulation and re-invokes KVM_RUN. Even if userspace makes use of another thread to emulate the DMA transfer such MMIO exits are disruptive to the vCPU and they may also be quite frequent if, for example, the vCPU is sending a sequence of short packets to the network device. By supporting poll() on coalesced buffer fds, userspace can have dedicated threads wait for new doorbell writes and avoid the performance hit of userspace exits on the main vCPU threads. Signed-off-by: Ilias Stamatis <ilstam@xxxxxxxxxx> --- virt/kvm/coalesced_mmio.c | 20 ++++++++++++++++++++ virt/kvm/coalesced_mmio.h | 1 + 2 files changed, 21 insertions(+)
Reviewed-by: Paul Durrant <paul@xxxxxxx>
