docs: hest: add new "etc/acpi_table_hest_addr" and update workflow

While the HEST layout didn't change, there are some internal
changes related to how offsets are calculated and how memory error
events are triggered.

Update specs to reflect such changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <e3e8bd92ce40d997c67ac1d4d973c0041b8f59fc.1758610789.git.mchehab+huawei@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/specs/acpi_hest_ghes.rst

@@ -89,12 +89,21 @@ Design Details
     addresses in the "error_block_address" fields with a pointer to the
     respective "Error Status Data Block" in the "etc/hardware_errors" blob.
 
-(8) QEMU defines a third and write-only fw_cfg blob which is called
-    "etc/hardware_errors_addr". Through that blob, the firmware can send back
-    the guest-side allocation addresses to QEMU. The "etc/hardware_errors_addr"
-    blob contains a 8-byte entry. QEMU generates a single WRITE_POINTER command
-    for the firmware. The firmware will write back the start address of
-    "etc/hardware_errors" blob to the fw_cfg file "etc/hardware_errors_addr".
+(8) QEMU defines a third and write-only fw_cfg blob to store the location
+    where the error block offsets, read ack registers and CPER records are
+    stored.
+
+    Up to QEMU 9.2, the location was at "etc/hardware_errors_addr", and
+    contains a GPA for the beginning of "etc/hardware_errors".
+
+    Newer versions place the location at "etc/acpi_table_hest_addr",
+    pointing to the GPA of the HEST table.
+
+    Using above mentioned 'fw_cfg' files, the firmware can send back the
+    guest-side allocation addresses to QEMU. They contain a 8-byte entry.
+    QEMU generates a single WRITE_POINTER command for the firmware. The
+    firmware will write back the start address of either "etc/hardware_errors"
+    or HEST table at the corresponding fw_cfg file.
 
 (9) When QEMU gets a SIGBUS from the kernel, QEMU writes CPER into corresponding
     "Error Status Data Block", guest memory, and then injects platform specific
@@ -105,8 +114,5 @@ Design Details
      kernel, on receiving notification, guest APEI driver could read the CPER error
      and take appropriate action.
 
-(11) kvm_arch_on_sigbus_vcpu() uses source_id as index in "etc/hardware_errors" to
-     find out "Error Status Data Block" entry corresponding to error source. So supported
-     source_id values should be assigned here and not be changed afterwards to make sure
-     that guest will write error into expected "Error Status Data Block" even if guest was
-     migrated to a newer QEMU.
+(11) kvm_arch_on_sigbus_vcpu() reports RAS errors via a SEA notifications,
+     when a SIGBUS event is triggered.