On Mon, 07 Aug 2023 17:00:58 +0300 Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote: > Is there a good reason why KVM doesn't expose APIC memslot to a > nested guest? While nested guest runs, the L1's APICv is "inhibited" > effectively anyway, so writes to this memslot should update APIC > registers and be picked up by APICv hardware when L1 resumes > execution. > > Since APICv alows itself to be inhibited due to other reasons, it > means that just like AVIC, it should be able to pick up arbitrary > changes to APIC registers which happened while it was inhibited, just > like AVIC does. > > I'll take a look at the code to see if APICv does this (I know AVIC's > code much better that APICv's) > > Is there a reproducer for this bug? > > Best regards, > Maxim Levitsky >From reading old commits (3a2936dedd20 and 1313cc2bd8f6), I interprete that current KVM implementation does not expect direct APIC access from L2 guests. I assume that there might be some challenging implementation issues. To reproduce the problem, we need to run a micro hypervisor named BitVisor on KVM. This hypervisor, when running on real machine, lets its guest access physical APIC directly. As BitVisor intends to run on real machine, when running under KVM, it conceals all KVM related features reported through CPUID. The L2 guest will initialize and run as if it runs on a physical machine. We also need an Intel machine that support APICv. (I test on Intel 13th machine. The problem should also be reproducible on Intel 12th machine). Current BitVisor's SVM implementations always monitor MMIO access so we cannot reproduce the problem. BitVisor VMX implementation under UEFI environment by default hooks the APIC access during initialization. The purpose of this APIC access hook is to bootstrap AP processors during UEFI ExitBootServices. When booting a guest OS, the firmware sends INIT signal during ExitBootServices. BitVisor then bootstrap AP processors, put them to guest mode, and unhook APIC access. After this, the guest can now access APIC memory directly. As far as I understand the KVM implemntation, when BitVisor still hooks APIC access, EPT_VIOLATION occurs when L2 guest accesses APIC page. The EPT_VIOLATION is then forwarded to BitVisor. BitVisor eventually accesses APIC on behalf of the L2 guest. In this case, APICv works properly because the access is from L1. After BitVisor unhooks the APIC page, the first access to APIC from the L2 guest goes to EPT_VIOLATION handling path. This handling path marks the APIC page with a reserved flag, and causes the access to retry eventually. Subsequent accesses are then handled in EPT_MISCONFIG path, emulating the MMIO access. Interrupt seems to disappear after this. Here is the steps to reproduce the problem. 1) hg clone http://hg.code.sf.net/p/bitvisor/code bitvisor-code 2) Enter the cloned directory and type 'make' (No need to worry about warnings at the moment. The default configuration is good enough to reproduce the problem). We now have bitvisor.elf after the compilation. 3) Enter boot/uefi-boot, and type 'make' to compile the UEFI bootloader. We need mingw for this. We now have loadvmm.efi after the compilation. 4) Put bitvisor.elf and loadvmm.efi to together in a folder. The folder is going to look like the following: ~/x86_test ├── bitvisor.elf └── loadvmm.efi 5) Run the following qemu command. Replace UEFI firmware path and other parameters as you prefer. Make sure -smp 2 is there. Otherwise, there will be no INIT signal during UEFI ExitBootServices. (I use QEMU 8.0.3) qemu-system-x86_64 -cpu host -enable-kvm -bios /usr/share/edk2-ovmf/OVMF_CODE.fd \ -drive file=fat:rw:~/x86_test/,format=raw \ -cdrom ~/Downloads/Fedora-Workstation-Live-x86_64-38-1.6.iso \ -M q35 -m 8192 -smp 2 -serial stdio 6) During the launch, enter the bios config by hitting esc key repeatedly. Then, select 'Boot Manager' and choose 'EFI Internel Shell' to enter the UEFI shell. 7) The directory we specify in the command should be at fs0. Type 'fs0:' in the shell. 8) Type 'loadvmm.efi' to load BitVisor. We should see the following message Loading ............................................................... Starting BitVisor... Copyright (c) 2007, 2008 University of Tsukuba All rights reserved. ACPI DMAR not found. FACS address 0x7FBDD000 Module not found. Processor 0 (BSP) ooooooooooooooooooooooooooooooooooooooooooooooooooo ... MCFG [0] 0000:00-FF (B0000000,10000000) Starting a virtual machine. 9) We should now return to the shell. Right now we are running as a L2 guest. 10) Next is to boot Linux from the live cd or your prefered method. We can see the panic related to "panic - not syncing: IO-APIC + timer doesn't work!". The panic can be reproduced quite easy. Even though, it happens to pass to timer check, or you specify 'no_timer_check' boot parameter, it will stall during SMP bringup. The idea from step 6 to step 10 is to start BitVisor first, and start Linux on top of it. You can adjust the step as you like. Feel free to ask me anything regarding reproducing the problem with BitVisor if the giving steps are not sufficient. The problem does not happen when enable_apicv=N. Note that SMP bringup with enable_apicv=N can fail. This is another problem. We don't have to worry about this for now. Linux seems to have no delay between INIT DEASSERT and SIPI during its SMP bringup. This can easily makes INIT and SIPI pending together resultling in signal lost. I admit that my knowledge on KVM and APICv is very limited. I may misunderstand the problem. If you don't mind, would it be possible for you to guide me which code path should I pay attention to? I would love to learn to find out the actual cause of the problem. Best Regards Ake Koomsin
