On 14.05.20 13:47, David Hildenbrand wrote: > On 14.05.20 13:10, David Hildenbrand wrote: >> On 14.05.20 12:12, David Hildenbrand wrote: >>> On 14.05.20 12:02, teawater wrote: >>>> >>>> >>>>> 2020年5月14日 16:48，David Hildenbrand <david@xxxxxxxxxx> 写道： >>>>> >>>>> On 14.05.20 08:44, teawater wrote: >>>>>> Hi David, >>>>>> >>>>>> I got a kernel warning with v2 and v3. >>>>> >>>>> Hi Hui, >>>>> >>>>> thanks for playing with the latest versions. Surprisingly, I can >>>>> reproduce even by hotplugging a DIMM instead as well - that's good, so >>>>> it's not related to virtio-mem, lol. Seems to be some QEMU setup issue >>>>> with older machine types. >>>>> >>>>> Can you switch to a newer qemu machine version, especially >>>>> pc-i440fx-5.0? Both, hotplugging DIMMs and virtio-mem works for me with >>>>> that QEMU machine just fine. >>>> >>>> I still could reproduce this issue with pc-i440fx-5.0 or pc. Did I miss anything? >>>> >>> >>> Below I don't even see virtio_mem. I had to repair the image (filesystem >>> fsck) because it was broken, can you try that as well? >>> >>> Also, it would be great if you could test with v4. >>> >> >> Correction, something seems to be broken either in QEMU or the kernel. Once I >> define a DIMM so it's added and online during boot, I get these issues: >> >> (I have virtio-mem v4 installed in the guest) >> >> #! /bin/bash >> sudo x86_64-softmmu/qemu-system-x86_64 \ >> -machine pc-i440fx-5.0,accel=kvm,usb=off \ >> -cpu host \ >> -no-reboot \ >> -nographic \ >> -device ide-hd,drive=hd \ >> -drive if=none,id=hd,file=/home/dhildenb/git/Fedora-Cloud-Base-31-1.9.x86_64.qcow2,format=qcow2 \ >> -m 1g,slots=10,maxmem=2G \ >> -smp 1 \ >> -object memory-backend-ram,id=mem0,size=256m \ >> -device pc-dimm,id=dimm0,memdev=mem0 \ >> -s \ >> -monitor unix:/var/tmp/monitor,server,nowait >> >> >> Without the DIMM it seems to work just fine. >> > > And another correction. > > Using QEMU v5.0.0, Linux 5.7-rc5, untouched > Fedora-Cloud-Base-32-1.6.x86_64.qcow2, I get even without any memory hotplug: > > #! /bin/bash > sudo x86_64-softmmu/qemu-system-x86_64 \ > -machine pc-i440fx-5.0,accel=kvm,usb=off \ > -cpu host \ > -no-reboot \ > -nographic \ > -device ide-hd,drive=hd \ > -drive if=none,id=hd,file=/home/dhildenb/git/Fedora-Cloud-Base-32-1.6.x86_64.qcow2,format=qcow2 \ > -m 5g,slots=10,maxmem=6G \ > -smp 1 \ > -s \ > -kernel /home/dhildenb/git/linux/arch/x86/boot/bzImage \ > -append "console=ttyS0 rd.shell nokaslr swiotlb=noforce" \ > -monitor unix:/var/tmp/monitor,server,nowait > > > Observe how big the initial RAM even is! > > > So this is no DIMM/hotplug/virtio_mem issue. With memory hotplug, it seems to get > more likely to trigger if "swiotlb=noforce" is not specified. > > "swiotlb=noforce" seems to trigger some pre-existing issue here. Without > "swiotlb=noforce", I was only able to observe this via pc-i440fx-2.1, > (talking to myself :) ) I think I finally understood why using "swiotlb=noforce" with hotplugged memory is wrong - or with memory > 3GB. Via "swiotlb=noforce" you tell the system to "Never use bounce buffers (for debugging)". This works as long as all memory is DMA memory (e.g., < 3GB) AFAIK. "If specified, trying to map memory that cannot be used with DMA will fail, and a rate-limited warning will be printed." Hotplugged memory (under QEMU) is never added below 4GB, because of the PCI hole. So both, memory from DIMMs and from virtio-mem will end up at or above 4GB. To make a device use that memory, you need bounce buffers. Hotplugged memory is never DMA memory. -- Thanks, David / dhildenb
