Add a platform portability blog post

Also fixes the docker_image bazel rule, and website-server make target.

Fixes #3273

PiperOrigin-RevId: 338606668

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+# Platform Portability
+
+Hardware virtualization is often seen as a requirement to provide an additional
+isolation layer for untrusted applications. However, hardware virtualization
+requires expensive bare-metal machines or cloud instances to run safely with
+good performance, increasing cost and complexity for Cloud users. gVisor,
+however, takes a more flexible approach.
+
+One of the pillars of gVisor's architecture is portability, allowing it to run
+anywhere that runs Linux. Modern Cloud-Native applications run in containers in
+many different places, from bare metal to virtual machines, and can't always
+rely on nested virtualization. It is important for gVisor to be able to support
+the environments where you run containers.
+
+gVisor achieves portability through an abstraction called a _Platform_.
+Platforms can have many implementations, and each implementation can cover
+different environments, making use of available software or hardware features.
+
+## Background
+
+Before we can understand how gVisor achieves portability using platforms, we
+should take a step back and understand how applications interact with their
+host.
+
+Container sandboxes can provide an isolation layer between the host and
+application by virtualizing one of the layers below it, including the hardware
+or operating system. Many sandboxes virtualize the hardware layer by running
+applications in virtual machines. gVisor takes a different approach by
+virtualizing the OS layer.
+
+When an application is run in a normal situation the host operating system loads
+the application into user memory and schedules it for execution. The operating
+system scheduler eventually schedules the application to a CPU and begins
+executing it. It then handles the application's requests, such as for memory and
+the lifecycle of the application. gVisor virtualizes these interactions, such as
+system calls, and context switching that happen between an application and OS.
+
+[System calls](https://en.wikipedia.org/wiki/System_call) allow applications to
+ask the OS to perform some task for it. System calls look like a normal function
+call in most programming languages though works a bit differently under the
+hood. When an application system call is encountered some special processing
+takes place to do a
+[context switch](https://en.wikipedia.org/wiki/Context_switch) into kernel mode
+and begin executing code in the kernel before returning a result to the
+application. Context switching may happen in other situations as well. For
+example, to respond to an interrupt.
+
+## The Platform Interface
+
+gVisor provides a sandbox which implements the Linux OS interface, intercepting
+OS interactions such as system calls and implements them in the sandbox kernel.
+
+It does this to limit interactions with the host, and protect the host from an
+untrusted application running in the sandbox. The Platform is the bottom layer
+of gVisor which provides the environment necessary for gVisor to control and
+manage applications. In general, the Platform must:
+
+1.  Provide the ability to create and manage memory address spaces.
+2.  Provide execution contexts for running applications in those memory address
+    spaces.
+3.  Provide the ability to change execution context and return control to gVisor
+    at specific times (e.g. system call, page fault)
+
+This interface is conceptually simple, but very powerful. Since the Platform
+interface only requires these three capabilities, it gives gVisor enough control
+for it to act as the application's OS, while still allowing the use of very
+different isolation technologies under the hood. You can learn more about the
+Platform interface in the
+[Platform Guide](https://gvisor.dev/docs/architecture_guide/platforms/).
+
+## Implementations of the Platform Interface
+
+While gVisor can make use of technologies like hardware virtualization, it
+doesn't necessarily rely on any one technology to provide a similar level of
+isolation. The flexibility of the Platform interface allows for implementations
+that use technologies other than hardware virtualization. This allows gVisor to
+run in VMs without nested virtualization, for example. By providing an
+abstraction for the underlying platform, each implementation can make various
+tradeoffs regarding performance or hardware requirements.
+
+Currently gVisor provides two gVisor Platform implementations; the Ptrace
+Platform, and the KVM Platform, each using very different methods to implement
+the Platform interface.
+
+![gVisor Platforms](../../../../../docs/architecture_guide/platforms/platforms.png "Platforms")
+
+The Ptrace Platform uses
+[PTRACE\_SYSEMU](http://man7.org/linux/man-pages/man2/ptrace.2.html) to trap
+syscalls, and uses the host for memory mapping and context switching. This
+platform can run anywhere that ptrace is available, which includes most Linux
+systems, VMs or otherwise.
+
+The KVM Platform uses virtualization, but in an unconventional way. gVisor runs
+in a virtual machine but as both guest OS and VMM, and presents no virtualized
+hardware layer. This provides a simpler interface that can avoid hardware
+initialization for fast start up, while taking advantage of hardware
+virtualization support to improve memory isolation and performance of context
+switching.
+
+The flexibility of the Platform interface allows for a lot of room to improve
+the existing KVM and ptrace platforms, as well as the ability to utilize new
+methods for improving gVisor's performance or portability in future Platform
+implementations.
+
+## Portability
+
+Through the Platform interface, gVisor is able to support bare metal, virtual
+machines, and Cloud environments while still providing a highly secure sandbox
+for running untrusted applications. This is especially important for Cloud and
+Kubernetes users because it allows gVisor to run anywhere that Kubernetes can
+run and provide similar experiences in multi-region, hybrid, multi-platform
+environments.
+
+Give gVisor's open source platforms a try. Using a Platform is as easy as
+providing the `--platform` flag to `runsc`. See the documentation on
+[changing platforms](https://gvisor.dev/docs/user_guide/platforms/) for how to
+use different platforms with Docker. We would love to hear about your experience
+so come chat with us in our
+[Gitter channel](https://gitter.im/gvisor/community), or send us an
+[issue on Github](https://gvisor.dev/issue) if you run into any problems.