On Wed, May 06, 2020 at 11:53:40AM +0100, Dr. David Alan Gilbert wrote: > * Peter Xu (pet...@redhat.com) wrote: > > System resets will also reset system memory layout. Although the memory > > layout > > after the reset should probably the same as before the reset, we still need > > to > > do frequent memory section removals and additions during the reset process. > > Those operations could accidentally lose per-mem-section information like > > KVM > > memslot dirty bitmaps. > > > > Previously we keep those dirty bitmaps by sync it during memory removal. > > However that's hard to make it right after all [1]. Instead, we sync dirty > > pages before system reset if we know we're during a precopy migration. This > > should solve the same problem explicitly. > > > > [1] https://lore.kernel.org/qemu-devel/20200327150425.GJ422390@xz-x1/ > > I think the problem is knowing whether this is sufficient or whether you > definitely need to do it for other cases of removal/readd.
Right that's the tricky part. I planned to look into the other potential cases after I get a feedback from Paolo or anyone that think this approach is acceptable, but of course I can even start to look into them now. When I said this should mostly be for reset, it comes from this commit message: commit 4cc856fabae1447d53890e707c70f257a7691174 Author: zhanghailiang <zhang.zhanghaili...@huawei.com> Date: Thu Apr 2 19:26:31 2015 +0000 kvm-all: Sync dirty-bitmap from kvm before kvm destroy the corresponding dirty_bitmap Sometimes, we destroy the dirty_bitmap in kvm_memory_slot before any sync action occur, this bit in dirty_bitmap will be missed, and which will lead the corresponding dirty pages to be missed in migration. This usually happens when do migration during VM's Start-up or Reboot. That's where the get-dirty-log was missing and Hailiang reported an issue with that probably on system reboot (but, again, I know nothing about the use case behind this...). Today I even tried to dig deeper into the initial place that this is introduced, and it's: commit 3fbffb628c001bd540dc9c1805bdf7aa8591da4d Author: Avi Kivity <a...@redhat.com> Date: Sun Jan 15 16:13:59 2012 +0200 kvm: flush the dirty log when unregistering a slot Otherwise, the dirty log information is lost in the kernel forever. Fixes opensuse-12.1 boot screen, which changes the vga windows rapidly. Signed-off-by: Avi Kivity <a...@redhat.com> So it's Avi's 8 years ago patch probably trying to fix a display issue since VGA tracks dirty bit without migration. IIUC this should belong to the case you raised the concern about "removing a device MMIO region with RAM backed". So I'll definitely look into that too, which I planned to, after all. > > However, assuming we need to do it during reset, how do we know this is > the right point to do it, and not something further inside the reset > process? Is this just on the basis that vcpus are stopped? "Stopped vcpus" is not the hint for "this is the right place", but definitely a good thing because we don't even need to consider multiple layers of dirty bit caches when they're stopped (for which the last layer is the PML, which is unluckily per-vcpu rather than per-memslot). It's just something nice and something extra we got. Even if vcpus are not stopped at this point (I believe the above VGA MMIO region removal could be the case where I need to take care of later, that we will have to collect dirty bits during the vcpus running), however as I mentioned in the other thread it should be a stack high enough to do some global dirty sync (because both dirty log and dirty ring need some global sync, and I believe that's one of the reasons why current code is still racy - we always need to flush the PML on every vcpu even for dirty log!). Sorry I went too far... should probably go back to the topic on "the right place" - I think this should be the right place for reset because AFAIK all the memslot add/remove is done inside device reset hooks, qemu_devices_reset() (or MachineClass.reset(), which could also change stuff, but finally calls qemu_devices_reset() too). I tried to verify this by enable the tracepoint at trace_kvm_set_user_memory() and also dump something at qemu_system_reset() before qemu_devices_reset(), I think it proves my understanding that all trace_kvm_set_user_memory() triggers after that point. Thanks, -- Peter Xu