On Tue, May 05, 2020 at 08:00:15PM -0700, Sean Christopherson wrote: > On Tue, May 05, 2020 at 10:32:15PM -0400, Qian Cai wrote: > > Today’s linux-next started to fail with this config, > > > > https://raw.githubusercontent.com/cailca/linux-mm/master/kcsan.config > > > > qemu-kvm-2.12.0-99.module+el8.2.0+5827+8c39933c.x86_64 > > > > I believe it was working yesterday. Before I bury myself bisecting it, does > > anyone have any thought? > > It reproduces for me as well with my vanilla config in a VM. I can debug > and/or bisect, should be quite quick in a VM. > > VM is bailing on the EPT Violation at the reset vector, i.e. on the very > first exit. Presumably KVM is incorrectly setting vmx->fail somewhere. The __FILL_RETURN_BUFFER in the VM-Exit path was recently modified and changed how it clobbered EFLAGS, which causes KVM to think VM-Enter failed. Commit 089dd8e53126 ("x86/speculation: Change FILL_RETURN_BUFFER to work with objtool") introduced the change, but this is really a bug in KVM. The VM-Exit path shouldn't rely on __FILL_RETURN_BUFFER to set EFLAGS to a specific state, i.e. EFLAGS was always being clobbered, it just happened to work before now. I'll get a patch sent out shortly.
