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+% Hypervisor FS
+% Revision 1
+
+\clearpage
+
+# Basics
+---------------- ---------------------
+ Status: **Supported**
+
+ Architectures: all
+
+ Components: Hypervisor, toolstack
+---------------- ---------------------
+
+# Overview
+
+The Hypervisor FS is a hierarchical name-value store for reporting
+information to guests, especially dom0. It is similar to the Linux
+kernel's sysfs. Entries and directories are created by the hypervisor,
+while the toolstack is able to use a hypercall to query the entry
+values or (if allowed by the hypervisor) to modify them.
+
+# User details
+
+With:
+
+ xenhypfs ls <path>
+
+the user can list the entries of a specific path of the FS. Using:
+
+ xenhypfs cat <path>
+
+the content of an entry can be retrieved. Using:
+
+ xenhypfs write <path> <string>
+
+a writable entry can be modified. With:
+
+ xenhypfs tree
+
+the complete Hypervisor FS entry tree can be printed.
+
+The FS paths are documented in `docs/misc/hypfs-paths.pandoc`.
+
+# Technical details
+
+Access to the hypervisor filesystem is done via the stable new hypercall
+__HYPERVISOR_filesystem_op. This hypercall supports a sub-command
+XEN_HYPFS_OP_get_version which will return the highest version of the
+interface supported by the hypervisor. Additions to the interface need
+to bump the interface version. The hypervisor is required to support the
+previous interface versions, too (this implies that additions will always
+require new sub-commands in order to allow the hypervisor to decide which
+version of the interface to use).
+
+* hypercall interface specification
+ * `xen/include/public/hypfs.h`
+* hypervisor internal files
+ * `xen/include/xen/hypfs.h`
+ * `xen/common/hypfs.c`
+* `libxenhypfs`
+ * `tools/libs/libxenhypfs/*`
+* `xenhypfs`
+ * `tools/misc/xenhypfs.c`
+* path documentation
+ * `docs/misc/hypfs-paths.pandoc`
+
+# Testing
+
+Any new parameters or hardware mitigations should be verified to show up
+correctly in the filesystem.
+
+# Areas for improvement
+
+* More detailed access rights
+* Entries per domain and/or per cpupool
+
+# Known issues
+
+* None
+
+# References
+
+* None
+
+# History
+
+------------------------------------------------------------------------
+Date Revision Version Notes
+---------- -------- -------- -------------------------------------------
+2020-01-23 1 Xen 4.14 Document written
+---------- -------- -------- -------------------------------------------
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+# Xenhypfs Paths
+
+This document attempts to define all the paths which are available
+in the Xen hypervisor file system (hypfs).
+
+The hypervisor file system can be accessed via the xenhypfs tool.
+
+## Notation
+
+The hypervisor file system is similar to the Linux kernel's sysfs.
+In this document directories are always specified with a trailing "/".
+
+The following notation conventions apply:
+
+ DIRECTORY/
+
+ PATH = VALUES [TAGS]
+
+The first syntax defines a directory. It normally contains related
+entries and the general scope of the directory is described.
+
+The second syntax defines a file entry containing values which are
+either set by the hypervisor or, if the file is writable, can be set
+by the user.
+
+PATH can contain simple regex constructs following the Perl compatible
+regexp syntax described in pcre(3) or perlre(1).
+
+A hypervisor file system entry name can be any 0-delimited byte string
+not containing any '/' character. The names "." and ".." are reserved
+for file system internal use.
+
+VALUES are strings and can take the following forms:
+
+* STRING -- an arbitrary 0-delimited byte string.
+* INTEGER -- An integer, in decimal representation unless otherwise
+ noted.
+* "a literal string" -- literal strings are contained within quotes.
+* (VALUE | VALUE | ... ) -- a set of alternatives. Alternatives are
+ separated by a "|" and all the alternatives are enclosed in "(" and
+ ")".
+* {VALUE, VALUE, ... } -- a list of possible values separated by "," and
+ enclosed in "{" and "}".
+
+Additional TAGS may follow as a comma separated set of the following
+tags enclosed in square brackets.
+
+* w -- Path is writable by the user. This capability is usually
+ limited to the control domain (e.g. dom0).
+* ARM | ARM32 | ARM64 | X86: the path is available for the respective
+ architecture only.
+* PV -- Path is valid for PV capable hypervisors only.
+* HVM -- Path is valid for HVM capable hypervisors only.
+* CONFIG_* -- Path is valid only in case the hypervisor was built with
+ the respective config option.
+
+So an entry could look like this:
+
+ /cpu-bugs/active-pv/xpti = ("No"|{"dom0", "domU", "PCID on"}) [w,X86,PV]
+
+Possible values would be "No" or a list of "dom0", "domU", and "PCID on".
+The entry would be writable and it would exist on X86 only and only if the
+hypervisor is configured to support PV guests.
+
+## Example
+
+A populated Xen hypervisor file system might look like the following example:
+
+ /
+ buildinfo/ directory containing build-time data
+ config contents of .config file used to build Xen
+ cpu-bugs/ x86: directory of cpu bug information
+ l1tf "Vulnerable" or "Not vulnerable"
+ mds "Vulnerable" or "Not vulnerable"
+ meltdown "Vulnerable" or "Not vulnerable"
+ spec-store-bypass "Vulnerable" or "Not vulnerable"
+ spectre-v1 "Vulnerable" or "Not vulnerable"
+ spectre-v2 "Vulnerable" or "Not vulnerable"
+ mitigations/ directory of mitigation settings
+ bti-thunk "N/A", "RETPOLINE", "LFENCE" or "JMP"
+ spec-ctrl "No", "IBRS+" or IBRS-"
+ ibpb "No" or "Yes"
+ l1d-flush "No" or "Yes"
+ md-clear "No" or "VERW"
+ l1tf-barrier "No" or "Yes"
+ active-hvm/ directory for mitigations active in hvm doamins
+ msr-spec-ctrl "No" or "Yes"
+ rsb "No" or "Yes"
+ eager-fpu "No" or "Yes"
+ md-clear "No" or "Yes"
+ active-pv/ directory for mitigations active in pv doamins
+ msr-spec-ctrl "No" or "Yes"
+ rsb "No" or "Yes"
+ eager-fpu "No" or "Yes"
+ md-clear "No" or "Yes"
+ xpti "No" or list of "dom0", "domU", "PCID on"
+ l1tf-shadow "No" or list of "dom0", "domU"
+ params/ directory with hypervisor parameter values
+ (boot/runtime parameters)
+
+## General Paths
+
+#### /
+
+The root of the hypervisor file system.
On the 2019 Xen developer summit there was agreement that the Xen hypervisor should gain support for a hierarchical name-value store similar to the Linux kernel's sysfs. In the beginning there should only be basic support: entries can be added from the hypervisor itself only, there is a simple hypercall interface to read the data. Add a feature document for setting the base of a discussion regarding the desired functionality and the entries to add. Signed-off-by: Juergen Gross <jgross@suse.com> --- V1: - remove the "--" prefixes of the sub-commands of the user tool (Jan Beulich) - rename xenfs to xenhypfs (Jan Beulich) - add "tree" and "write" options to user tool V2: - move example tree to the paths description (Ian Jackson) - specify allowed characters for keys and values (Ian Jackson) V3: - correct introduction (writable entries) V4: - add list specification - add entry example (Julien Grall) - correct date and Xen version (Julien Grall) - add ARM64 as possible architecture (Julien Grall) - add version description to the feature doc (Jan Beulich) --- docs/features/hypervisorfs.pandoc | 92 +++++++++++++++++++++++++++++++++ docs/misc/hypfs-paths.pandoc | 105 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 197 insertions(+) create mode 100644 docs/features/hypervisorfs.pandoc create mode 100644 docs/misc/hypfs-paths.pandoc