Hi Alex, > From: Alex Bennée <alex.ben...@linaro.org> > Sent: Wednesday, September 4, 2024 4:46 PM > To: Salil Mehta <salil.me...@huawei.com> > > Salil Mehta <salil.me...@huawei.com> writes: > > > Hi Alex, > > > >> -----Original Message----- > >> From: Alex Bennée <alex.ben...@linaro.org> > >> Sent: Thursday, August 29, 2024 11:00 AM > >> To: Gustavo Romero <gustavo.rom...@linaro.org> > >> > >> Gustavo Romero <gustavo.rom...@linaro.org> writes: > >> > >> > Hi Salil, > >> > > >> > On 6/13/24 8:36 PM, Salil Mehta via wrote: > >> <snip> > >> >> (VI) Commands Used > >> >> ================== > >> >> A. Qemu launch commands to init the machine: > >> >> $ qemu-system-aarch64 --enable-kvm -machine virt,gic-version=3 > \ > >> >> -cpu host -smp cpus=4,maxcpus=6 \ > >> >> -m 300M \ > >> >> -kernel Image \ > >> >> -initrd rootfs.cpio.gz \ > >> >> -append "console=ttyAMA0 root=/dev/ram rdinit=/init > maxcpus=2 > >> acpi=force" \ > >> >> -nographic \ > >> >> -bios QEMU_EFI.fd \ > >> >> B. Hot-(un)plug related commands: > >> >> # Hotplug a host vCPU (accel=kvm): > >> >> $ device_add host-arm-cpu,id=core4,core-id=4 > >> >> # Hotplug a vCPU (accel=tcg): > >> >> $ device_add cortex-a57-arm-cpu,id=core4,core-id=4 > >> > > >> > Since support for hotplug is disabled on TCG, remove these two > >> lines > in v4 cover letter? > >> > >> Why is it disabled for TCG? We should aim for TCG being as close to > >> KVM as possible for developers even if it is not a production solution. > > > > Agreed In principle. Yes, that would be of help. > > > > > > Context why it was disabled although most code to support TCG exist: > > > > I had reported a crash in the RFC V1 (June 2020) about TCGContext > > counter overflow assertion during repeated hot(un)plug operation. > > Miguel from Oracle was able to reproduce this problem last year in Feb > > and also suggested a fix but he later found out in his testing that there > was > a problem during migration. > > > > RFC V1 June 2020: > > https://lore.kernel.org/qemu-devel/20200613213629.21984-1- > salil.mehta@ > > huawei.com/ > > Scroll to below: > > [...] > > THINGS TO DO: > > (*) Migration support > > (*) TCG/Emulation support is not proper right now. Works to a certain > extent > > but is not complete. especially the unrealize part in which there is a > > overflow of tcg contexts. The last is due to the fact tcg maintains a > > count on number of context(per thread instance) so as we hotplug the > vcpus > > this counter keeps on incrementing. But during hot-unplug the counter > is > > not decremented. > > Right so the translation cache is segmented by vCPU to support parallel JIT > operations. The easiest solution would be to ensure we dimension for the > maximum number of vCPUs, which it should already, see > tcg_init_machine(): > > unsigned max_cpus = ms->smp.max_cpus; > ... > tcg_init(s->tb_size * MiB, s->splitwx_enabled, max_cpus);
Agreed. We have done that and have a patch for that as well. But it is still a work-in-progress and I've lost context a bit. https://github.com/salil-mehta/qemu/commit/107cf5ca7cf3716bc0f8c68e98e1da3939f449ce For now, I've very quickly tried to enable and run the TCG to gain back the context. I've now hit a different problem during TCG vCPU unrealization phase, while pthread_join() waits on halt condition variable for MTTCG vCPU thread to exit, there is a crash somewhere. Look like some race condition. Will dig this further. Best regards Salil. > > @ Feb 2023, [Linaro-open-discussions] Re: Qemu TCG support for > > virtual-cpuhotplug/online-policy > > > > https://op-lists.linaro.org/archives/list/linaro-open-discussions@op-l > > ists.linaro.org/message/GMDFTEZE6WUUI7LZAYOWLXFHAPXLCND5/ > > > > Last status reported by Miguel was that there was problem with the TCG > > and he intended to fix this. He was on paternity leave so I will try to > gather > the exact status of the TCG today. > > > > Thanks > > Salil > > > > > >> > >> -- > >> Alex Bennée > >> Virtualisation Tech Lead @ Linaro > > -- > Alex Bennée > Virtualisation Tech Lead @ Linaro
