Messages in this thread | | | Date | Fri, 17 Feb 2023 18:57:54 +0000 | Subject | Re: [RFC PATCH v3 08/31] KVM: selftests: Require GCC to realign stacks on function entry | From | Ackerley Tng <> |
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> > 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 https://lore.kernel.org/lkml/32866e5d00174697730d6231d2fb81f6b8d98c8a.1676659352.git.ackerleytng@google.com/
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