diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index ba5bd1e502..6912762f28 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -1924,6 +1924,9 @@
   <ioapic driver='qemu'/>
   <hpt resizing='required'/>
   <vmcoreinfo state='on'/>
+  <smm state='on'>
+    <tseg unit='MiB'>48</tseg>
+  </smm>
 </features>
 ...</pre>
 
@@ -2079,10 +2082,55 @@
           <span class="since">Since 1.2.16</span>
       </dd>
       <dt><code>smm</code></dt>
-      <dd>Depending on the <code>state</code> attribute (values <code>on</code>,
+      <dd>
+        <p>
+          Depending on the <code>state</code> attribute (values <code>on</code>,
           <code>off</code>, default <code>on</code>) enable or disable
           System Management Mode.
           <span class="since">Since 2.1.0</span>
+        </p><p> Optional sub-element <code>tseg</code> can be used to specify
+          the amount of memory dedicated to SMM's extended TSEG. That offers a
+          fourth option size apart from the existing ones (1 MiB, 2 MiB and 8
+          MiB) that the guest OS (or rather loader) can choose from. The size
+          can be specified as a value of that element, optional attribute
+          <code>unit</code> can be used to specify the unit of the
+          aforementioned value (defaults to 'MiB').  If set to 0 the extended
+          size is not advertised and only the default ones (see above) are
+          available.
+        </p><p>
+          <b>If the VM is booting you should leave this option alone, unless you
+          are very certain you know what you are doing.</b>
+        </p><p>
+          This value is configurable due to the fact that the calculation cannot
+          be done right with the guarantee that it will work correctly.  In
+          QEMU, the user-configurable extended TSEG feature was unavailable up
+          to and including <code>pc-q35-2.9</code>. Starting with
+          <code>pc-q35-2.10</code> the feature is available, with default size
+          16 MiB.  That should suffice for up to roughly 272 VCPUs, 5 GiB guest
+          RAM in total, no hotplug memory range, and 32 GiB of 64-bit PCI MMIO
+          aperture.  Or for 48 VCPUs, with 1TB of guest RAM, no hotplug DIMM
+          range, and 32GB of 64-bit PCI MMIO aperture. The values may also vary
+          based on the loader the VM is using.
+        </p><p>
+          Additional size might be needed for significantly higher VCPU counts
+          or increased address space (that can be memory, maxMemory, 64-bit PCI
+          MMIO aperture size; roughly 8 MiB of TSEG per 1 TiB of address space)
+          which can also be rounded up.
+        </p><p>
+          Due to the nature of this setting being similar to "how much RAM
+          should the guest have" users are advised to either consult the
+          documentation of the guest OS or loader (if there is any), or test
+          this by trial-and-error changing the value until the VM boots
+          successfully.  Yet another guiding value for users might be the fact
+          that 48 MiB should be enough for pretty large guests (240 VCPUs and
+          4TB guest RAM), but it is on purpose not set as default as 48 MiB of
+          unavailable RAM might be too much for small guests (e.g. with 512 MiB
+          of RAM).
+        </p><p>
+          See <a href="#elementsMemoryAllocation">Memory Allocation</a>
+          for more details about the <code>unit</code> attribute.
+          <span class="since">Since 4.5.0</span> (QEMU only)
+        </p>
       </dd>
       <dt><code>ioapic</code></dt>
       <dd>Tune the I/O APIC. Possible values for the
diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
index c5c8c575b8..550fb10159 100644
--- a/docs/schemas/domaincommon.rng
+++ b/docs/schemas/domaincommon.rng
@@ -4846,6 +4846,11 @@
                 <attribute name="state">
                   <ref name="virOnOff"/>
                 </attribute>
+                <optional>
+                  <element name="tseg">
+                    <ref name="scaledInteger"/>
+                  </element>
+                </optional>
               </optional>
             </element>
           </optional>
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 5be773cda4..ab93bb7b45 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -19898,6 +19898,19 @@ virDomainDefParseXML(xmlDocPtr xml,
         VIR_FREE(nodes);
     }
 
+    if (def->features[VIR_DOMAIN_FEATURE_SMM] == VIR_TRISTATE_SWITCH_ON) {
+        int rv = virDomainParseScaledValue("string(./features/smm/tseg)",
+                                           "string(./features/smm/tseg/@unit)",
+                                           ctxt,
+                                           &def->tseg_size,
+                                           1024 * 1024, /* Defaults to mebibytes */
+                                           ULLONG_MAX,
+                                           false);
+        if (rv < 0)
+            goto error;
+        def->tseg_specified = rv;
+    }
+
     if ((n = virXPathNodeSet("./features/capabilities/*", ctxt, &nodes)) < 0)
         goto error;
 
@@ -22020,6 +22033,33 @@ virDomainDefFeaturesCheckABIStability(virDomainDefPtr src,
         }
     }
 
+    /* smm */
+    if (src->features[VIR_DOMAIN_FEATURE_SMM] == VIR_TRISTATE_SWITCH_ON) {
+        if (src->tseg_specified != dst->tseg_specified) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("SMM TSEG differs: source: %s, destination: '%s'"),
+                           src->tseg_specified ? _("specified") : _("not specified"),
+                           dst->tseg_specified ? _("specified") : _("not specified"));
+            return false;
+        }
+
+        if (src->tseg_specified &&
+            src->tseg_size != dst->tseg_size) {
+            const char *unit_src, *unit_dst;
+            unsigned long long short_size_src = virFormatIntPretty(src->tseg_size,
+                                                                   &unit_src);
+            unsigned long long short_size_dst = virFormatIntPretty(dst->tseg_size,
+                                                                   &unit_dst);
+
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("Size of SMM TSEG size differs: "
+                             "source: '%llu %s', destination: '%llu %s'"),
+                           short_size_src, unit_src,
+                           short_size_dst, unit_dst);
+            return false;
+        }
+    }
+
     return true;
 }
 
@@ -27446,7 +27486,6 @@ virDomainDefFormatInternal(virDomainDefPtr def,
             case VIR_DOMAIN_FEATURE_PMU:
             case VIR_DOMAIN_FEATURE_PVSPINLOCK:
             case VIR_DOMAIN_FEATURE_VMPORT:
-            case VIR_DOMAIN_FEATURE_SMM:
                 switch ((virTristateSwitch) def->features[i]) {
                 case VIR_TRISTATE_SWITCH_LAST:
                 case VIR_TRISTATE_SWITCH_ABSENT:
@@ -27463,6 +27502,31 @@ virDomainDefFormatInternal(virDomainDefPtr def,
 
                 break;
 
+            case VIR_DOMAIN_FEATURE_SMM:
+                if (def->features[i] != VIR_TRISTATE_SWITCH_ABSENT) {
+                    virTristateSwitch state = def->features[i];
+                    virBuffer attrBuf = VIR_BUFFER_INITIALIZER;
+                    virBuffer childBuf = VIR_BUFFER_INITIALIZER;
+
+                    virBufferAsprintf(&attrBuf, " state='%s'",
+                                      virTristateSwitchTypeToString(state));
+
+                    if (state == VIR_TRISTATE_SWITCH_ON &&
+                        def->tseg_specified) {
+                        const char *unit;
+                        unsigned long long short_size = virFormatIntPretty(def->tseg_size,
+                                                                           &unit);
+
+                        virBufferSetChildIndent(&childBuf, buf);
+                        virBufferAsprintf(&childBuf, "<tseg unit='%s'>%llu</tseg>\n",
+                                          unit, short_size);
+                    }
+
+                    virXMLFormatElement(buf, "smm", &attrBuf, &childBuf);
+                }
+
+                break;
+
             case VIR_DOMAIN_FEATURE_APIC:
                 if (def->features[i] == VIR_TRISTATE_SWITCH_ON) {
                     virBufferAddLit(buf, "<apic");
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 8a8121bf83..a41cdce6ff 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -2421,6 +2421,9 @@ struct _virDomainDef {
     char *hyperv_vendor_id;
     int apic_eoi;
 
+    bool tseg_specified;
+    unsigned long long tseg_size;
+
     virDomainClockDef clock;
 
     size_t ngraphics;
diff --git a/tests/genericxml2xmlindata/tseg.xml b/tests/genericxml2xmlindata/tseg.xml
new file mode 100644
index 0000000000..49483f874c
--- /dev/null
+++ b/tests/genericxml2xmlindata/tseg.xml
@@ -0,0 +1,23 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219100</memory>
+  <currentMemory unit='KiB'>219100</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='q35'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <features>
+    <smm state='on'>
+      <tseg unit='MiB'>48</tseg>
+    </smm>
+  </features>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+  </devices>
+</domain>
diff --git a/tests/genericxml2xmltest.c b/tests/genericxml2xmltest.c
index d8270a6cae..daad6e0f78 100644
--- a/tests/genericxml2xmltest.c
+++ b/tests/genericxml2xmltest.c
@@ -141,6 +141,8 @@ mymain(void)
     DO_TEST_FULL("cachetune-colliding-types", false, true,
                  TEST_COMPARE_DOM_XML2XML_RESULT_FAIL_PARSE);
 
+    DO_TEST("tseg");
+
     virObjectUnref(caps);
     virObjectUnref(xmlopt);