docs/system: Add SGX documentation to the system manual

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-34-yang.zhong@intel.com>
[Convert to reStructuredText, and adopt the standard === --- ~~~ headings
 suggested for example by Linux. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/system/i386/sgx.rst

+Software Guard eXtensions (SGX)
+===============================
+
+Overview
+--------
+
+Intel Software Guard eXtensions (SGX) is a set of instructions and mechanisms
+for memory accesses in order to provide security accesses for sensitive
+applications and data. SGX allows an application to use it's pariticular
+address space as an *enclave*, which is a protected area provides confidentiality
+and integrity even in the presence of privileged malware. Accesses to the
+enclave memory area from any software not resident in the enclave are prevented,
+including those from privileged software.
+
+Virtual SGX
+-----------
+
+SGX feature is exposed to guest via SGX CPUID. Looking at SGX CPUID, we can
+report the same CPUID info to guest as on host for most of SGX CPUID. With
+reporting the same CPUID guest is able to use full capacity of SGX, and KVM
+doesn't need to emulate those info.
+
+The guest's EPC base and size are determined by Qemu, and KVM needs Qemu to
+notify such info to it before it can initialize SGX for guest.
+
+Virtual EPC
+~~~~~~~~~~~
+
+By default, Qemu does not assign EPC to a VM, i.e. fully enabling SGX in a VM
+requires explicit allocation of EPC to the VM. Similar to other specialized
+memory types, e.g. hugetlbfs, EPC is exposed as a memory backend.
+
+SGX EPC is enumerated through CPUID, i.e. EPC "devices" need to be realized
+prior to realizing the vCPUs themselves, which occurs long before generic
+devices are parsed and realized.  This limitation means that EPC does not
+require -maxmem as EPC is not treated as {cold,hot}plugged memory.
+
+Qemu does not artificially restrict the number of EPC sections exposed to a
+guest, e.g. Qemu will happily allow you to create 64 1M EPC sections. Be aware
+that some kernels may not recognize all EPC sections, e.g. the Linux SGX driver
+is hardwired to support only 8 EPC sections.
+
+The following Qemu snippet creates two EPC sections, with 64M pre-allocated
+to the VM and an additional 28M mapped but not allocated::
+
+ -object memory-backend-epc,id=mem1,size=64M,prealloc=on \
+ -object memory-backend-epc,id=mem2,size=28M \
+ -M sgx-epc.0.memdev=mem1,sgx-epc.1.memdev=mem2
+
+Note:
+
+The size and location of the virtual EPC are far less restricted compared
+to physical EPC. Because physical EPC is protected via range registers,
+the size of the physical EPC must be a power of two (though software sees
+a subset of the full EPC, e.g. 92M or 128M) and the EPC must be naturally
+aligned.  KVM SGX's virtual EPC is purely a software construct and only
+requires the size and location to be page aligned. Qemu enforces the EPC
+size is a multiple of 4k and will ensure the base of the EPC is 4k aligned.
+To simplify the implementation, EPC is always located above 4g in the guest
+physical address space.
+
+Migration
+~~~~~~~~~
+
+Qemu/KVM doesn't prevent live migrating SGX VMs, although from hardware's
+perspective, SGX doesn't support live migration, since both EPC and the SGX
+key hierarchy are bound to the physical platform. However live migration
+can be supported in the sense if guest software stack can support recreating
+enclaves when it suffers sudden lose of EPC; and if guest enclaves can detect
+SGX keys being changed, and handle gracefully. For instance, when ERESUME fails
+with #PF.SGX, guest software can gracefully detect it and recreate enclaves;
+and when enclave fails to unseal sensitive information from outside, it can
+detect such error and sensitive information can be provisioned to it again.
+
+CPUID
+~~~~~
+
+Due to its myriad dependencies, SGX is currently not listed as supported
+in any of Qemu's built-in CPU configuration. To expose SGX (and SGX Launch
+Control) to a guest, you must either use `-cpu host` to pass-through the
+host CPU model, or explicitly enable SGX when using a built-in CPU model,
+e.g. via `-cpu <model>,+sgx` or `-cpu <model>,+sgx,+sgxlc`.
+
+All SGX sub-features enumerated through CPUID, e.g. SGX2, MISCSELECT,
+ATTRIBUTES, etc... can be restricted via CPUID flags. Be aware that enforcing
+restriction of MISCSELECT, ATTRIBUTES and XFRM requires intercepting ECREATE,
+i.e. may marginally reduce SGX performance in the guest. All SGX sub-features
+controlled via -cpu are prefixed with "sgx", e.g.::
+
+  $ qemu-system-x86_64 -cpu help | xargs printf "%s\n" | grep sgx
+  sgx
+  sgx-debug
+  sgx-encls-c
+  sgx-enclv
+  sgx-exinfo
+  sgx-kss
+  sgx-mode64
+  sgx-provisionkey
+  sgx-tokenkey
+  sgx1
+  sgx2
+  sgxlc
+
+The following Qemu snippet passes through the host CPU but restricts access to
+the provision and EINIT token keys::
+
+ -cpu host,-sgx-provisionkey,-sgx-tokenkey
+
+SGX sub-features cannot be emulated, i.e. sub-features that are not present
+in hardware cannot be forced on via '-cpu'.
+
+Virtualize SGX Launch Control
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Qemu SGX support for Launch Control (LC) is passive, in the sense that it
+does not actively change the LC configuration.  Qemu SGX provides the user
+the ability to set/clear the CPUID flag (and by extension the associated
+IA32_FEATURE_CONTROL MSR bit in fw_cfg) and saves/restores the LE Hash MSRs
+when getting/putting guest state, but Qemu does not add new controls to
+directly modify the LC configuration.  Similar to hardware behavior, locking
+the LC configuration to a non-Intel value is left to guest firmware.  Unlike
+host bios setting for SGX launch control(LC), there is no special bios setting
+for SGX guest by our design. If host is in locked mode, we can still allow
+creating VM with SGX.
+
+Feature Control
+~~~~~~~~~~~~~~~
+
+Qemu SGX updates the `etc/msr_feature_control` fw_cfg entry to set the SGX
+(bit 18) and SGX LC (bit 17) flags based on their respective CPUID support,
+i.e. existing guest firmware will automatically set SGX and SGX LC accordingly,
+assuming said firmware supports fw_cfg.msr_feature_control.
+
+Launching a guest
+-----------------
+
+To launch a SGX guest:
+
+.. parsed-literal::
+
+  |qemu_system_x86| \\
+   -cpu host,+sgx-provisionkey \\
+   -object memory-backend-epc,id=mem1,size=64M,prealloc=on \\
+   -object memory-backend-epc,id=mem2,size=28M \\
+   -M sgx-epc.0.memdev=mem1,sgx-epc.1.memdev=mem2
+
+Utilizing SGX in the guest requires a kernel/OS with SGX support.
+The support can be determined in guest by::
+
+  $ grep sgx /proc/cpuinfo
+
+and SGX epc info by::
+
+  $ dmesg | grep sgx
+  [    1.242142] sgx: EPC section 0x180000000-0x181bfffff
+  [    1.242319] sgx: EPC section 0x181c00000-0x1837fffff
+
+References
+----------
+
+- `SGX Homepage <https://software.intel.com/sgx>`__
+
+- `SGX SDK <https://github.com/intel/linux-sgx.git>`__
+
+- SGX specification: Intel SDM Volume 3

docs/system/target-i386.rst

@@ -26,6 +26,7 @@ Architectural features
    :maxdepth: 1
 
    i386/cpu
+   i386/sgx
 
 .. _pcsys_005freq: