On Mon, Sep 14, 2020 at 04:38:13PM -0500, Josh Poimboeuf wrote: > On Mon, Sep 14, 2020 at 02:07:19PM -0700, Sean Christopherson wrote: > > > RSP needs to be aligned to what? How would this align the stack, other > > > than by accident? > > > > Ah, yeah, that's lacking info. > > > > 16-byte aligned to correctly mimic CPU behavior when vectoring an IRQ/NMI. > > When not changing stack, the CPU aligns RSP before pushing the frame. > > > > The above shenanigans work because the x86-64 ABI also requires RSP to be > > 16-byte aligned prior to CALL. RSP is thus 8-byte aligned due to CALL > > pushing the return IP, and so creating the stack frame by pushing RBP makes > > it 16-byte aliagned again. > > As Uros mentioned, the kernel doesn't do this.
Argh, apparently I just got lucky with my compiles then. I added explicit checks on RSP being properly aligned and thought that confirmed the kernel played nice. Bummer.
