Re: [RFC PATCH v3 08/31] KVM: selftests: Require GCC to realign stacks on function entry

Ackerley Tng
2023-02-17T18:58:03Z

> > I figured it out!
> >
> > GCC assumes that the stack is 16-byte aligned **before** the call
> > instruction. Since call pushes rip to the stack, GCC will compile code
> > assuming that on entrance to the function, the stack is -8 from a
> > 16-byte aligned address.
> >
> > Since for TDs we do a ljmp to guest code, providing a function's
> > address, the stack was not modified by a call instruction pushing rip to
> > the stack, so the stack is 16-byte aligned when the guest code starts
> > running, instead of 16-byte aligned -8 that GCC expects.
> >
> > For VMs, we set rip to a function pointer, and the VM starts running
> > with a 16-byte algined stack too.
> >
> > To fix this, I propose that in vm_arch_vcpu_add(), we align the
> > allocated stack address and then subtract 8 from that:
> >
> > @@ -573,10 +573,13 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm
> *vm,
> > uint32_t vcpu_id,
> > vcpu_init_cpuid(vcpu, kvm_get_supported_cpuid());
> > vcpu_setup(vm, vcpu);
> >
> > + stack_vaddr += (DEFAULT_STACK_PGS * getpagesize());
> > + stack_vaddr = ALIGN_DOWN(stack_vaddr, 16) - 8;

> The ALIGN_DOWN should be unnecessary, we've got larger issues if
> getpagesize() isn't
> 16-byte aligned and/or if __vm_vaddr_alloc() returns anything but a
> page-aligned
> address. Maybe add a TEST_ASSERT() sanity check that stack_vaddr is
> page-aligned
> at this point?

> And in addition to the comment suggested by Maciej, can you also add a
> comment
> explaining the -8 adjust? Yeah, someone can go read the changelog, but I
> think
> this is worth explicitly documenting in code.

> Lastly, can you post it as a standalone patch?

> Many thanks!

Thanks Maciej and Sean, I've made the changes you requested and posted
it as a standalone patch at