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SubjectRe: KVM overflows the stack
 > Yes, things like kvm_lapic_state are way too big to be on the stack.

I had a quick look at the code, and my worry about dynamic allocation
would be that handling allocation failure seems like it might get
tricky. Eg for handling struct kvm_pv_mmu_op_buffer (which is 528 bytes
on the stack in kvm_pv_mmu_op()) can you deal with an mmu op failing?
(maybe in that case you can easily by just setting *ret to 0?)

> There's an additional problem here, that apparently your gcc (which
> version?) doesn't fold objects in a switch statement into the same
> stack slot:
> switch (...) {
> case x: {
> struct medium a;
> ...
> }
> case y:
> struct medium b;
> ...
> }
> };

A trick for this is to do:

union {
struct medium1 a;
struct medium2 b;
} u;

switch (...) {
case x:
use u.a;

case y:
use u.b;

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