GDB qXfer target description for real-mode qemu

target.xml

<?xml version="1.0"?>
<!-- Copyright (C) 2010-2017 Free Software Foundation, Inc.

     Copying and distribution of this file, with or without modification,
     are permitted in any medium without royalty provided the copyright
     notice and this notice are preserved.  -->

<!DOCTYPE target SYSTEM "gdb-target.dtd">
<target>
    <architecture>i8086</architecture>
    <feature name="org.gnu.gdb.i386.core">
        <flags id="i386_eflags" size="4">
            <field name="CF" start="0" end="0"/>
            <field name="" start="1" end="1"/>
            <field name="PF" start="2" end="2"/>
            <field name="AF" start="4" end="4"/>
            <field name="ZF" start="6" end="6"/>
            <field name="SF" start="7" end="7"/>
            <field name="TF" start="8" end="8"/>
            <field name="IF" start="9" end="9"/>
            <field name="DF" start="10" end="10"/>
            <field name="OF" start="11" end="11"/>
            <field name="NT" start="14" end="14"/>
            <field name="RF" start="16" end="16"/>
            <field name="VM" start="17" end="17"/>
            <field name="AC" start="18" end="18"/>
            <field name="VIF" start="19" end="19"/>
            <field name="VIP" start="20" end="20"/>
            <field name="ID" start="21" end="21"/>
        </flags>

        <reg name="eax" bitsize="32" type="int32"/>
        <reg name="ecx" bitsize="32" type="int32"/>
        <reg name="edx" bitsize="32" type="int32"/>
        <reg name="ebx" bitsize="32" type="int32"/>
        <reg name="esp" bitsize="32" type="data_ptr"/>
        <reg name="ebp" bitsize="32" type="data_ptr"/>
        <reg name="esi" bitsize="32" type="int32"/>
        <reg name="edi" bitsize="32" type="int32"/>

        <reg name="eip" bitsize="32" type="code_ptr"/>
        <reg name="eflags" bitsize="32" type="i386_eflags"/>
        <reg name="cs" bitsize="32" type="int32"/>
        <reg name="ss" bitsize="32" type="int32"/>
        <reg name="ds" bitsize="32" type="int32"/>
        <reg name="es" bitsize="32" type="int32"/>
        <reg name="fs" bitsize="32" type="int32"/>
        <reg name="gs" bitsize="32" type="int32"/>

        <reg name="st0" bitsize="80" type="i387_ext"/>
        <reg name="st1" bitsize="80" type="i387_ext"/>
        <reg name="st2" bitsize="80" type="i387_ext"/>
        <reg name="st3" bitsize="80" type="i387_ext"/>
        <reg name="st4" bitsize="80" type="i387_ext"/>
        <reg name="st5" bitsize="80" type="i387_ext"/>
        <reg name="st6" bitsize="80" type="i387_ext"/>
        <reg name="st7" bitsize="80" type="i387_ext"/>

        <reg name="fctrl" bitsize="32" type="int" group="float"/>
        <reg name="fstat" bitsize="32" type="int" group="float"/>
        <reg name="ftag" bitsize="32" type="int" group="float"/>
        <reg name="fiseg" bitsize="32" type="int" group="float"/>
        <reg name="fioff" bitsize="32" type="int" group="float"/>
        <reg name="foseg" bitsize="32" type="int" group="float"/>
        <reg name="fooff" bitsize="32" type="int" group="float"/>
        <reg name="fop" bitsize="32" type="int" group="float"/>
    </feature>
    <feature name="org.gnu.gdb.i386.32bit.sse">
        <vector id="v4f" type="ieee_single" count="4"/>
        <vector id="v2d" type="ieee_double" count="2"/>
        <vector id="v16i8" type="int8" count="16"/>
        <vector id="v8i16" type="int16" count="8"/>
        <vector id="v4i32" type="int32" count="4"/>
        <vector id="v2i64" type="int64" count="2"/>
        <union id="vec128">
            <field name="v4_float" type="v4f"/>
            <field name="v2_double" type="v2d"/>
            <field name="v16_int8" type="v16i8"/>
            <field name="v8_int16" type="v8i16"/>
            <field name="v4_int32" type="v4i32"/>
            <field name="v2_int64" type="v2i64"/>
            <field name="uint128" type="uint128"/>
        </union>
        <flags id="i386_mxcsr" size="4">
            <field name="IE" start="0" end="0"/>
            <field name="DE" start="1" end="1"/>
            <field name="ZE" start="2" end="2"/>
            <field name="OE" start="3" end="3"/>
            <field name="UE" start="4" end="4"/>
            <field name="PE" start="5" end="5"/>
            <field name="DAZ" start="6" end="6"/>
            <field name="IM" start="7" end="7"/>
            <field name="DM" start="8" end="8"/>
            <field name="ZM" start="9" end="9"/>
            <field name="OM" start="10" end="10"/>
            <field name="UM" start="11" end="11"/>
            <field name="PM" start="12" end="12"/>
            <field name="FZ" start="15" end="15"/>
        </flags>

        <reg name="xmm0" bitsize="128" type="vec128" regnum="32"/>
        <reg name="xmm1" bitsize="128" type="vec128"/>
        <reg name="xmm2" bitsize="128" type="vec128"/>
        <reg name="xmm3" bitsize="128" type="vec128"/>
        <reg name="xmm4" bitsize="128" type="vec128"/>
        <reg name="xmm5" bitsize="128" type="vec128"/>
        <reg name="xmm6" bitsize="128" type="vec128"/>
        <reg name="xmm7" bitsize="128" type="vec128"/>

        <reg name="mxcsr" bitsize="32" type="i386_mxcsr" group="vector"/>
    </feature>
</target>

Solution from @x746e works for me too

qemu:
QEMU emulator version 6.1.0
gdb:
GNU gdb (GDB) 10.2