OvmfPkg/VirtHstiDxe: add README.md

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>

OvmfPkg/VirtHstiDxe/README.md

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+
+# virtual machine platform hsti driver
+
+This driver supports three tests.
+
+## VIRT_HSTI_BYTE0_SMM_SMRAM_LOCK
+
+Verify the SMM memory is properly locked down.
+
+Supported platforms:
+ * Qemu Q35 (SMM_REQUIRE=TRUE builds).
+
+## VIRT_HSTI_BYTE0_SMM_SECURE_VARS_FLASH
+
+Verify the variable store is not writable for normal (not SMM) code.
+
+Supported platforms:
+ * Qemu Q35 (SMM_REQUIRE=TRUE builds).
+
+## VIRT_HSTI_BYTE0_READONLY_CODE_FLASH
+
+Verify the firmware code is not writable for the guest.
+
+Supported platforms:
+ * Qemu Q35
+ * Qemu PC
+
+# qemu flash configuration
+
+With qemu being configured properly flash behavior should be this:
+
+configuration                  |  OVMF_CODE.fd  |  OVMF_VARS.fd
+-------------------------------|----------------|---------------
+SMM_REQUIRE=TRUE, SMM mode     |  read-only     |  writable
+SMM_REQUIRE=TRUE, normal mode  |  read-only (1) |  read-only (2)
+SMM_REQUIRE=FALSE              |  read-only (3) |  writable
+
+VIRT_HSTI_BYTE0_READONLY_CODE_FLASH will verify (1) + (3).
+VIRT_HSTI_BYTE0_SMM_SECURE_VARS_FLASH will verify (2).
+
+## qemu command line for SMM_REQUIRE=TRUE builds
+```
+qemu-system-x86-64 -M q35,smm=on,pflash0=code,pflash1=vars \
+  -blockdev node-name=code,driver=file,filename=OVMF_CODE.fd,read-only=on \
+  -blockdev node-name=vars,driver=file,filename=OVMF_VARS.fd \
+  -global driver=cfi.pflash01,property=secure,value=on \
+  [ ... more options here ... ]
+```