Which CPUID features are “hidden” or “masked” from the native hardware in KVM implementation. Why does the KVM hypervisor hide / mask
such features?
Related
we are trying to port android as guest on Xen hypervisor in raspberry pi4 hardware.
What changes need to be done in kernel of Guest OS(Android) for achieving paravirtualization on XEN hypervisor?
is there any reference I can follow?
thanks
Hello Humble Stackoverflow users,
According to this XEN article, PVH mode keeps all components paravirtualized except Pagetables - "but instead of requiring PV MMU (often called paravirtualized page tables), it uses the HVM hardware extensions to virtualize the pagetables".
Accoring to link i've provided above and wiki, you can learn that Pagetables are heavily connected with CPU workflow. However, following article sais that HVM hardware Extensions are providing CPU virtualization - "Technically speaking, HVM refers to a set of extensions that make it much simpler to virtualize one component: the processor."
At the end all this left me in confused state.
Does XEN PVH mode requires Intel VT-x or AMD-V HW virtualization or not?
Regards,
Leshcat
pvh only works with intel vt for now and there is no support for 32bit guests yet, as you can see in the readme
http://xenbits.xen.org/docs/unstable/misc/pvh-readme.txt
Following remain to be done for PVH: AMD port.
https://blog.xenproject.org/2015/01/15/less-is-more-in-the-new-xen-project-4-5-release/
PVH initial domain support for Intel has been added and now supports running as dom0 and FreeBSD with Linux platforms
I recently have acquired a Apple G5 computer (PPC 970) and am interested in learning more about the PowerPC architecture (most of my systems programming knowledge comes from x86 and my own hobby kernel).
After using the computer a while and getting used to PowerPC assembly (RISC), I noticed that low level CPU virtualization is not possible on PowerPC 970 based Macs. The CPU in documentation (PowerPC 64) seems to support hypervisor mode, but it has been noted that it is not possible due to Open Firmware.
Do all operating systems which are loaded from Open Firmware on PowerPC 970 series Macs load in hypervisor mode, making "nested" virtualization impossible? If this is true, why does Open Firmware load all Operating systems in hypervisor mode? Is this in order to provide a secure layer for communication between the the Operating System and Open Firmware (using firmware for everything except ACPI and memory discovery during boot, which requires a transition into "real-mode", is unsafe in x86?).
Also if the Operating system were using hyper-calls to facilitate a secure transition to firmware based routines, wouldn't this impose a large penalty just as syscalls do?
I'm not privy to Apple's hardware designs, but I've heard that the HV mode (ie., HV=1 in the Machine State Register) was disabled, through hardware, on the CPUs used in the G5 machines.
If this is the case, then it's not up to the system firmware to enable/disable HV mode - it's simply not available.
At the time that these machines were available, other Power hardware designs had a small amount of firmware running in HV=1 mode, and only exposed HV=0 to the kernel. However, the G5 wasn't one of these.
I am really new to virtualization , i have read in some places that hardware virtualization is also referred as hardware assisted virtualization , is it true or false.
From my understanding hardware virtualization means Hardware itself has some embedded software which is helpful in managing systems resources between OS , is it right.
And if possible tell me what are the different types of Virtualization?
Different types of virtualization
In the traditional x86 architecture, operating system kernels expect direct CPU access running in Ring 0, which is the most privileged level.
Software Virtualization
With software virtualization, guest operating systems cannot run in Ring 0 because the VMM sits there. The guest operating systems must therefore run in Ring 1, but there's a catch: Some x86 instructions work only in Ring 0, so the operating systems must be recompiled to avoid them. This process is called paravirtualization, and it is impractical — especially if the source code for the OS is not available. To get around this, VMMs traps these instructions and emulates them, which unfortunately results in an enormous performance hit: Virtual machines can be significantly slower than real physical ones.
Hardware Assisted Virtualization
Thus, Intel and AMD have introduced their new virtualization technologies, a handful of new instructions and — crucially — a new privilege level. The hypervisor can now run at "Ring -1"; so the guest operating systems can run in Ring 0. There's no need for paravirtualization, the VMM does less work, and the performance hit is reduced
Wiki puts it very nicely
Virtualization
The term "virtualization" was coined in the 1960s to refer to a virtual machine (sometimes called "pseudo machine"), a term which itself dates from the experimental IBM M44/44X system. The creation and management of virtual machines has been called "platform virtualization", or "server virtualization", more recently.
Platform virtualization is performed on a given hardware platform by host software (a control program), which creates a simulated computer environment, a virtual machine (VM), for its guest software. The guest software is not limited to user applications; many hosts allow the execution of complete operating systems. The guest software executes as if it were running directly on the physical hardware, with several notable caveats. Access to physical system resources (such as the network access, display, keyboard, and disk storage) is generally managed at a more restrictive level than the host processor and system-memory. Guests are often restricted from accessing specific peripheral devices, or may be limited to a subset of the device's native capabilities, depending on the hardware access policy implemented by the virtualization host.
Hardware-assisted virtualization
In hardware-assisted virtualization, the hardware provides architectural support that facilitates building a virtual machine monitor and allows guest OSes to be run in isolation. Hardware-assisted virtualization was first introduced on the IBM System/370 in 1972, for use with VM/370, the first virtual machine operating system. In 2005 and 2006, Intel and AMD provided additional hardware to support virtualization. Sun Microsystems (now Oracle Corporation) added similar features in their UltraSPARC T-Series processors in 2005. Examples of virtualization platforms adapted to such hardware include Linux KVM, VMware Workstation, VMware Fusion, Microsoft Hyper-V, Microsoft Virtual PC, Xen, Parallels Desktop for Mac, Oracle VM Server for SPARC, VirtualBox and Parallels Workstation.
Also refer to the following links for more explanations:
http://searchvmware.techtarget.com/definition/hardware-virtualization
http://www.anandtech.com/show/2480
Modern hardware-assisted desktop virtualization products (like VMWare Workstation or VirtualBox) normally provide the guest OS with a virtual graphics adapter that has a limited functionality.
Is it possible to switch the adapters, i.e provide guest OS with direct access to the real graphics adapter, and assign a virtual graphics adapter to the host OS? Is there any software that has this functionality? If not, is it possible to develop such system? Let's assume we only have a single guest OS.
It should be possible soon with using VGA passthrough as implemented by Xen 4 (unstable branch for now):
Quoting the Xen FAQ:
"Xen 4.0.0 is the first version to support VGA graphics adapter passthrough to Xen HVM (fully virtualized) guests. This means you can give HVM guest full and direct control of the graphics adapter, making it possible to have high performance full 3D and video acceleration in a virtual machine"
"Xen VGA passthrough requires IOMMU (Intel VT-d) support from the motherboard chipset, from the motherboard BIOS and from Xen."
Note that only a few motherboard support IOMMU for now. See the FAQ for more info.
I/O hardware Virtualization especially for graphics card is made using technology called IOMMU.
AMD has published a specification for IOMMU technology in the HyperTransport architecture. Intel has published a specification for IOMMU technology as Virtualization Technology for Directed I/O, abbreviated VT-d.
With virtualization, guest operating systems can use hardware that is not specifically made for virtualization. An example for IOMMU is Graphics Address Remapping Table (GART) used by AGP and PCI Express graphics cards. Higher performance hardware such as graphics cards use DMA to access memory directly; in a virtual environment all the memory addresses are remapped by the virtual machine software, which causes DMA devices to fail. The IOMMU handles this remapping, allowing for the native device drivers to be used in a guest operating system.
Most of the Virtualization softwares supports hardware acceleration for OpenGL and some of them provides experimental Direct3D acceleration such as VMWare. Products from VMware, Citrix and VirtualBox etc. provides hardware accelaration
What processor?
This is the idea behind I/O virtualization (Intel's implementation is called VT-d). You need CPU support to allow the guest direct access to the video hardware while blocking it from stomping on other resources, such as the disk system.