On 12/12/19 08:36, Michael S. Tsirkin wrote: > On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote: >>>> I'd say it won't be a big issue on locking 1/2M of host mem for a >>>> vm... >>>> Also note that if dirty ring is enabled, I plan to evaporate the >>>> dirty_bitmap in the next post. The old kvm->dirty_bitmap takes >>>> $GUEST_MEM/32K*2 mem. E.g., for 64G guest it's 64G/32K*2=4M. If with >>>> dirty ring of 8 vcpus, that could be 64K*8=0.5M, which could be even >>>> less memory used. >>> >>> Right - I think Avi described the bitmap in kernel memory as one of >>> design mistakes. Why repeat that with the new design? >> >> Do you have a source for that? > > Nope, it was a private talk. > >> At least the dirty bitmap has to be >> accessed from atomic context so it seems unlikely that it can be moved >> to user memory. > > Why is that? We could surely do it from VCPU context? Spinlock is taken. >> The dirty ring could use user memory indeed, but it would be much harder >> to set up (multiple ioctls for each ring? what to do if userspace >> forgets one? etc.). > > Why multiple ioctls? If you do like virtio packed ring you just need the > base and the size. You have multiple rings, so multiple invocations of one ioctl. Paolo
