Doubt regarding memory allocation in KVM

From: Shivank Garg <shivankgarg98@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Doubt regarding memory allocation in KVM
Date: Tue, 20 Apr 2021 11:15:45 +0530	[thread overview]
Message-ID: <CAOVCmzH4XEGMGgOpvnLU7_qW93cNit4yvb6kOV2BZNZH_8POJg@mail.gmail.com> (raw)

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??
(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?

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)

3. How/where can I access the GPFNs in the host? Is "gfn_t gfn = gpa
>> PAGE_SHIFT" and "gpa_t cr2_or_gpa" in the KVM page fault handler,
x86 is the same as GPFN. (that is can I use pfn_to_page in guest VM to
access the struct page in Guest)

Thank You.

Best Regards,
Shivank Garg
M.Tech Student,
IIT Kanpur