Subversion Repositories HelenOS-doc

<?xml version="1.0" encoding="UTF-8"?>

<appendix id="archspecs">

  <?dbhtml filename="arch.html"?>

  <title>Architecture Specific Notes</title>

  <section>

    <title>AMD64/Intel EM64T</title>

    <para>The amd64 architecture is a 64-bit extension of the older ia32

    architecture. Only 64-bit applications are supported. Creating this port

    was relatively easy, because it shares a lot of common code with ia32

    platform. However, the 64-bit extension has some specifics, which made the

    porting interesting.</para>

    <section>

      <title>Virtual Memory</title>

      <para>The amd64 architecture uses standard processor defined 4-level

      page mapping of 4KB pages. The NX(no-execute) flag on individual pages

      is fully supported.</para>

    </section>

    <section>

      <title>TLB-only Paging</title>

      <para>All memory on the amd64 architecture is memory mapped, if the

      kernel needs to access physical memory, a mapping must be created.

      During boot process the boot loader creates mapping for the first 20MB

      of physical memory. To correctly initialize the page mapping system, an

      identity mapping of whole physical memory must be created. However, to

      create the mapping it is unavoidable to allocate new - possibly unmapped

      - frames from frame allocator. The ia32 solves it by mapping first 2GB

      memory during boot process. The same solution on 64-bit platform becomes

      unfeasible because of the size of the possible address space.</para>

      <para>As soon as the exception routines are initialized, a special page

      fault exception handler is installed which provides a complete view of

      physical memory until the real page mapping system is initialized. It

      dynamically changes the page tables to always contain exactly the

      faulting address. The page then becomes cached in the TLB and on the

      next page fault the same tables can be utilized to handle another

      mapping.</para>

    </section>

    <section>

      <title>Mapping of Physical Memory</title>

      <para>The amd64 ABI document describes several modes of program layout.

      The operating system kernel should be compiled in a

      <emphasis>kernel</emphasis> mode - the kernel is located in the negative

      2 gigabytes (0xffffffff80000000-0xfffffffffffffffff) and can access data

      anywhere in the 64-bit space. This wouldn't allow kernel to see directly

      more than 2GB of physical memory. HelenOS duplicates the virtual mapping

      of the physical memory starting at 0xffff800000000000 and accesses all

      external references using this address range.</para>

    </section>

    <section>

      <title>Thread Local Storage</title>

      <para>The code accessing thread local storage uses a segment register FS

      as a base. The thread local storage is stored in the hidden 64-bit part

      of the FS register which must be written using priviledged machine

      specific instructions. Special syscall to change this register is

      provided to user applications. The TLS address for this platform is

      expected to point just after the end of the thread local data. The

      application sometimes need to get a real address of the thread local

      data in its address space but it is impossible to read the base of the

      FS segmentation register. The solution is to add the self-reference

      address to the end of thread local data, so that the application can

      read the address as %gs:0.</para>

      <figure float="1">

        <title>IA-32 &amp; AMD64 TLD</title>

        <mediaobject id="tldia32">

          <imageobject role="pdf">

            <imagedata fileref="images/tld_ia32.pdf" format="PDF" />

          </imageobject>

          <imageobject role="html">

            <imagedata fileref="images/tld_ia32.png" format="PNG" />

          </imageobject>

          <imageobject role="fop">

            <imagedata fileref="images/tld_ia32.svg" format="SVG" />

          </imageobject>

        </mediaobject>

      </figure>

    </section>

    <section>

      <title>Fast SYSCALL/SYSRET Support</title>

      <para>The entry point for system calls was traditionally a speed problem

      on the ia32 architecture. The amd64 supports SYSCALL/SYSRET

      instructions. Upon encountering the SYSCALL instruction, the processor

      changes privilege mode and transfers control to an address stored in

      machine specific register. Unlike other similar instructions it does not

      change stack to a known kernel stack, which must be done by the syscall

      entry routine. A hidden part of a GS register is provided to support the

      entry routine with data needed for switching to kernel stack.</para>

    </section>

    <section>

      <title>Debugging Support</title>

      <para>To provide developers tools for finding bugs, hardware breakpoints

      and watchpoints are supported. The kernel also supports self-debugging -

      it sets watchpoints on certain data and upon every modification

      automatically checks whether a correct value was written. It is

      worthwhile to mention, that since this feature was implemented, the

      watchpoint was never fired.</para>

    </section>

  </section>

  <section>

    <title>Intel IA-32</title>

    <para>The ia32 architecture uses 4K pages and processor supported 2-level

    page tables. Along with amd64 It is one of the 2 architectures that fully

    supports SMP configurations. The architecture is mostly similar to amd64,

    it even shares a lot of code. The debugging support is the same as with

    amd64. The thread local storage uses GS register.</para>

  </section>

  <section>

    <title>32-bit MIPS</title>

    <para>Both little and big endian kernels are supported. In order to test

    different page sizes, the mips32 page size was set to 16K. The mips32

    architecture is TLB-only, the kernel simulates 2-level page tables. On

    processors that support it, lazy FPU context switching is

    implemented.</para>

    <section>

      <title>Thread Local Storage</title>

      <para>The thread local storage support in compilers is a relatively

      recent phenomena. The standardization of such support for the mips32

      platform is very new and even the newest versions of GCC cannot generate

      100% correct code. Because of some weird MIPS processor variants, it was

      decided, that the TLS pointer will be gathered not from some of the free

      registers, but a special instruction was devised and the kernel is

      supposed to emulate it. HelenOS expects that the TLS pointer is in the

      K1 register. Upon encountering the reserved instruction exception and

      checking that the application is requesting a TLS pointer, it returns

      the contents of the K1 register. The K1 register is expected to point

      0x7000 bytes after the beginning of the thread local data.</para>

      <figure float="1">

        <title>MIPS &amp; PowerPC TLD</title>

        <mediaobject id="tldmips">

          <imageobject role="pdf">

            <imagedata fileref="images/tld_mips.pdf" format="PDF" />

          </imageobject>

          <imageobject role="html">

            <imagedata fileref="images/tld_mips.png" format="PNG" />

          </imageobject>

          <imageobject role="fop">

            <imagedata fileref="images/tld_mips.svg" format="SVG" />

          </imageobject>

        </mediaobject>

      </figure>

    </section>

    <section>

      <title>Lazy FPU Context Switching</title>

      <para>Implementing lazy FPU switching on MIPS architecture is

      straightforward. When coprocessor CP1 is disabled, any FPU intruction

      raises a Coprocessor Unusable exception. The generic lazy FPU context

      switch is then called that takes care of the correct context

      save/restore.</para>

    </section>

  </section>

  <section>

    <title>Power PC</title>

    <para>PowerPC allows kernel to enable mode, where data and intruction

    memory reads are not translated through virtual memory mapping

    (<emphasis>real mode</emphasis>). The real mode is automatically enabled

    when an exception occurs. However, the kernel uses the same memory

    structure as on other 32-bit platforms - physical memory is mapped into

    the top 2GB, userspace memory is available in the bottom half of the

    32-bit address space.</para>

    <section>

      <title>OpenFirmware Boot</title>

      <para>The OpenFirmware loads an image of HelenOS operating system and

      passes control to the HelenOS specific boot loader. The boot loader then

      performs following tasks:</para>

      <itemizedlist>

        <listitem>

          <para>Fetches information from OpenFirmware regarding memory

          structure, device information etc.</para>

        </listitem>

        <listitem>

          <para>Switches memory mapping to the real mode.</para>

        </listitem>

        <listitem>

          <para>Copies the kernel to proper physical address.</para>

        </listitem>

        <listitem>

          <para>Creates basic memory mapping and switches to the new kernel

          mapping, in which the kernel can run.</para>

        </listitem>

        <listitem>

          <para>Passes control to the kernel <function>main_bsp</function>

          function.</para>

        </listitem>

      </itemizedlist>

    </section>

    <section>

      <title>Thread Local Storage</title>

      <para>The Power PC thread local storage uses R2 register to hold an

      address, that is 0x7000 bytes after the beginning of the thread local

      data. Overally it is the same as on the MIPS architecture.</para>

    </section>

  </section>

  <section>

    <title>IA-64</title>

    <para>The ia64 kernel uses 16K pages.</para>

    <section>

      <title>Two IA-64 Stacks</title>

      <para>The architecture makes use of a pair of stacks. One stack is the

      ordinary memory stack while the other is a special register stack. This

      makes the ia64 architecture unique. HelenOS on ia64 solves the problem

      by allocating two physical memory frames for thread and scheduler

      stacks. The upper frame is used by the register stack while the first

      frame is used by the conventional memory stack. The generic kernel and

      userspace code had to be adjusted to cope with the possibility of

      allocating more frames for the stack.</para>

    </section>

    <section>

      <title>Thread Local Storage</title>

      <para>Although thread local storage is not officially supported in

      statically linked binaries, GCC supports it without any major obstacles.

      The r13 register is used as a thread pointer, the thread local data

      section starts at address r13+16.</para>

      <para><figure float="1">

          <title>IA-64 TLD</title>

          <mediaobject id="tldia64">

            <imageobject role="pdf">

              <imagedata fileref="images/tld_ia64.pdf" format="PDF" />

            </imageobject>

            <imageobject role="html">

              <imagedata fileref="images/tld_ia64.png" format="PNG" />

            </imageobject>

            <imageobject role="fop">

              <imagedata fileref="images/tld_ia64.svg" format="SVG" />

            </imageobject>

          </mediaobject>

        </figure></para>

    </section>

  </section>

</appendix>