From: Paolo Bonzini <pbonzini@redhat.com>
To: Shivank Garg <shivankgarg98@gmail.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: Re: Doubt regarding memory allocation in KVM
Date: Tue, 20 Apr 2021 08:52:00 +0200 [thread overview]
Message-ID: <e7c9b569-a1c4-7b8f-ce47-8e3526464c60@redhat.com> (raw)
In-Reply-To: <CAOVCmzH4XEGMGgOpvnLU7_qW93cNit4yvb6kOV2BZNZH_8POJg@mail.gmail.com>
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
prev parent reply other threads:[~2021-04-20 6:52 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-20 5:45 Doubt regarding memory allocation in KVM Shivank Garg
2021-04-20 6:52 ` Paolo Bonzini [this message]
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