| 534,7 → 534,7 |
|---|
| |
| <para>The remote TLB invalidation is called TLB shootdown. HelenOS uses |
| a simplified variant of the algorithm described in <xref |
| linkend="Black89" />. </para> |
| linkend="Black89" />.</para> |
| |
| <para>TLB shootdown is performed in three phases.</para> |
| |
| 572,85 → 572,71 |
|---|
| operation.</para> |
| </formalpara> |
| </section> |
| </section> |
| |
| <section> |
| <title>Address spaces</title> |
| <section> |
| <title>Address spaces</title> |
| |
| <section> |
| <indexterm> |
| <primary>address space</primary> |
| <para>last bits about address spaces</para> |
| |
| <secondary>- area</secondary> |
| </indexterm> |
| <section> |
| <indexterm> |
| <primary>address space</primary> |
| |
| <title>Address space areas</title> |
| <secondary>- ASID</secondary> |
| </indexterm> |
| |
| <para>Each address space consists of mutually disjunctive continuous |
| address space areas. Address space area is precisely defined by its |
| base address and the number of frames/pages is contains.</para> |
| <title>Address Space ID (ASID)</title> |
| |
| <para>Address space area , that define behaviour and permissions on |
| the particular area. <itemizedlist> |
| <listitem><emphasis>AS_AREA_READ</emphasis> flag indicates reading |
| permission.</listitem> |
| <para>Modern processor architectures optimize TLB utilization by |
| associating TLB entries with address spaces through assigning |
| identification numbers to them. In HelenOS, the term ASID, originally |
| taken from the mips32 terminology, is used to refer to the address space |
| identification number. The advantage of having ASIDs is that TLB does |
| not have to be invalidated on thread context switch as long as ASIDs are |
| unique. Unfotunatelly, architectures supported by HelenOS use all |
| different widths of ASID numbers<footnote> |
| <para>amd64 and ia32 don't use similar abstraction at all, mips32 |
| has 8-bit ASIDs and ia64 can have ASIDs between 18 to 24 bits |
| wide.</para> |
| </footnote> out of which none is sufficient. The amd64 and ia32 |
| architectures cannot make use of ASIDs as their TLB doesn't recognize |
| such an abstraction. Other architectures have support for ASIDs, but for |
| instance ppc32 doesn't make use of them in the current version of |
| HelenOS. The rest of the architectures does use ASIDs. However, even on |
| the ia64 architecture, the minimal supported width of ASID<footnote> |
| <para>RID in ia64 terminology.</para> |
| </footnote> is insufficient to provide a unique integer identifier to |
| all address spaces that might hypothetically coexist in the running |
| system. The situation on mips32 is even worse: the architecture has only |
| 256 unique identifiers.</para> |
| |
| <listitem><emphasis>AS_AREA_WRITE</emphasis> flag indicates |
| writing permission.</listitem> |
| <indexterm> |
| <primary>address space</primary> |
| |
| <listitem><emphasis>AS_AREA_EXEC</emphasis> flag indicates code |
| execution permission. Some architectures do not support execution |
| persmission restriction. In this case this flag has no |
| effect.</listitem> |
| <secondary>- ASID stealing</secondary> |
| </indexterm> |
| |
| <listitem><emphasis>AS_AREA_DEVICE</emphasis> marks area as mapped |
| to the device memory.</listitem> |
| </itemizedlist></para> |
| |
| <para>Kernel provides possibility tasks create/expand/shrink/share its |
| address space via the set of syscalls.</para> |
| </section> |
| |
| <section> |
| <indexterm> |
| <primary>address space</primary> |
| |
| <secondary>- ASID</secondary> |
| </indexterm> |
| |
| <title>Address Space ID (ASID)</title> |
| |
| <para>Every task in the operating system has it's own view of the |
| virtual memory. When performing context switch between different |
| tasks, the kernel must switch the address space mapping as well. As |
| modern processors perform very aggressive caching of virtual mappings, |
| flushing the complete TLB on every context switch would be very |
| inefficient. To avoid such performance penalty, some architectures |
| introduce an address space identifier, which allows storing several |
| different mappings inside TLB.</para> |
| |
| <para>HelenOS kernel can take advantage of this hardware support by |
| having an ASID abstraction. I.e. on ia64 kernel ASID is derived from |
| RID (region identifier) and on the mips32 kernel ASID is actually the |
| hardware identifier. As expected, this ASID information record is the |
| part of <emphasis>as_t</emphasis> structure.</para> |
| |
| <para>Due to the hardware limitations, hardware ASID has limited |
| length from 8 bits on ia64 to 24 bits on mips32, which makes it |
| impossible to use it as unique address space identifier for all tasks |
| running in the system. In such situations special ASID stealing |
| algoritm is used, which takes ASID from inactive task and assigns it |
| to the active task.</para> |
| |
| <indexterm> |
| <primary>address space</primary> |
| |
| <secondary>- ASID stealing</secondary> |
| </indexterm> |
| |
| <para> |
| <classname>ASID stealing algoritm here.</classname> |
| </para> |
| </section> |
| <para>To mitigate the shortage of ASIDs, HelenOS uses the following |
| strategy. When the system initializes, a FIFO queue<footnote> |
| <para>Note that architecture-specific measures are taken to avoid |
| too large FIFO queue. For instance, seven consecutive ia64 RIDs are |
| grouped to form one HelenOS ASID.</para> |
| </footnote> is created and filled with all available ASIDs. Moreover, |
| every address space remembers the number of processors on which it is |
| active. Address spaces that have a valid ASID and that are not active on |
| any processor are appended to the list of inactive address spaces with |
| valid ASID. When an address space needs to be assigned a valid ASID, it |
| first checks the FIFO queue. If it contains at least one ASID, the ASID |
| is allocated. If the queue is empty, an ASID is simply stolen from the |
| first address space in the list. In that case, the address space that |
| loses the ASID in favor of another address space, is removed from the |
| list. After the new ASID is purged from all TLBs, it can be used by the |
| address space. Note that this approach works due to the fact that the |
| number of ASIDs is greater than the maximal number of processors |
| supported by HelenOS and that there can be only one active address space |
| per processor. In other words, when the FIFO queue is empty, there must |
| be address spaces that are not active on any processor.</para> |
| </section> |
| </section> |
| </chapter> |