On 20/04/21 07:45, Shivank Garg wrote:
Hi, I'm learning about qemu KVM, looking into code and experimenting on it. I have the following doubts regarding it, I would be grateful if you help me to get some idea on them. 1. I observe that KVM allocates memory to guests when it needs it but doesn't take it back (except for ballooning case). Also, the Qemu/KVM process does not free the memory even when the guest is rebooted. In this case, Does the Guest VM get access to memory already pre-filled with some garbage from the previous run??
Yes.
(Since the host would allocate zeroed pages to guests the first time it requests but after that it's up to guests). Can it be a security issue?
No, it's the same that happens on non-virtual machine.
2. How does the KVM know if GPFN (guest physical frame number) is backed by an actual machine frame number in host? If not mapped, then it faults in the host and allocates a physical frame for guests in the host. (kvm_mmu_page_fault)
It's all handled by Linux. KVM only does a call to get_user_pages. See functions whose name starts with hva_to_pfn in virt/kvm/kvm_main.c
Given a GPA, the GFN is simply the guest physical address minus bits 0:11, so shifted right by 12.
Paolo
