linux-kernel - Re: [BUG net-next] arch/x86/kernel/cpu/bugs.c:2935: "Unpatched return thunk in use. This should not happen!" [STACKTRACE]

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Date: Tue, 2 Apr 2024 08:38:56 -0500
From: Michael Roth <michael.roth@....com>
To: <bp@...nel.org>
CC: <bgardon@...gle.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<dmatlack@...gle.com>, <hpa@...or.com>, <jpoimboe@...nel.org>,
	<kvm@...r.kernel.org>, <leitao@...ian.org>, <linux-kernel@...r.kernel.org>,
	<maz@...nel.org>, <mingo@...hat.com>, <mirsad.todorovac@....unizg.hr>,
	<pawan.kumar.gupta@...ux.intel.com>, <pbonzini@...hat.com>,
	<peterz@...radead.org>, <seanjc@...gle.com>, <shahuang@...hat.com>,
	<tabba@...gle.com>, <tglx@...utronix.de>, <x86@...nel.org>
Subject: Re: [BUG net-next] arch/x86/kernel/cpu/bugs.c:2935: "Unpatched
 return thunk in use. This should not happen!" [STACKTRACE]

On Tue, Apr 02, 2024 at 12:15:49PM +0200, bp@...nel.org wrote:
> From: Borislav Petkov <bp@...en8.de>
> 
> Sorry if this comes out weird - mail troubles currently.
> 
> On Thu, Mar 28, 2024 at 07:38:30AM -0500, Michael Roth wrote:
> > I'm seeing it pretty consistently on kvm/next as well. Not sure if
> > there's anything special about my config but starting a fairly basic
> > SVM guest seems to be enough to trigger it for me on the first
> > invocation of svm_vcpu_run().
> 
> Hmm, can you share your config and what exactly you're doing?
> 
> I can't reproduce with Mirsad's reproducer, probably because of .config
> differences. I tried making all CONFIG*KVM* options =y but no
> difference.

I've reproduced against tip/master from today and attached the host
config I used.

I can reproduce with a normal SVM guest using the following cmdline,
but I don't think there's anything particular special regarding what
QEMU options you use. It seems to trigger on the very first entry into
VMRUN path:

  /home/mroth/qemu-build-snp-v4-wip2/qemu-system-x86_64
    -smp 32,maxcpus=255 -cpu EPYC-Milan-v2 -overcommit cpu-pm=off
    -enable-kvm -m 4G,slots=5,maxmem=210G -vga std -nographic
    -machine pc,memory-backend=ram1
    -object memory-backend-memfd,id=ram1,size=4G,share=true,prealloc=false,reserve=false
    -device virtio-scsi-pci,id=scsi0,disable-legacy=on,iommu_platform=true
    -drive file=/home/mroth/ubuntu-18.04-seves2.qcow2,if=none,id=drive0,snapshot=on
    -device scsi-hd,id=hd0,drive=drive0,bus=scsi0.0
    -device virtio-net-pci,mac=52:54:00:6c:3c:01,netdev=netdev0,id=net0,disable-legacy=on,iommu_platform=true,romfile=
    -netdev tap,script=/home/mroth/qemu-ifup,id=netdev0
    -L /home/mroth/AMDSEV/snp-release-2024-02-22/usr/local/share/qemu
    -msg timestamp=on
    -drive if=pflash,format=raw,unit=0,file=/home/mroth/AMDSEV/snp-release-2024-02-22/usr/local/share/qemu/OVMF_CODE.fd,readonly=on
    -drive if=pflash,format=raw,unit=1,file=/home/mroth/AMDSEV/snp-release-2024-02-22/usr/local/share/qemu/OVMF_VARS.fd

I can also trigger using one of the more basic KVM selftests:

  make INSTALL_HDR_PATH="$headers_dir" headers_install
  make -C tools/testing/selftests TARGETS="kvm" EXTRA_CFLAGS="-DDEBUG -I$headers_dir"
  sudo tools/testing/selftests/kvm/userspace_io_test

after that you need to kexec or reboot to see the WARN_ONCE() again.

-Mike

> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette
> 

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