linux-kernel - Re: KVM overflows the stack

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Date:	Wed, 16 Jul 2008 23:08:01 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm@...r.kernel.org>,
	"Anthony N. Liguori \[imap\]" <aliguori@...ibm.com>
Subject: Re: 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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