> > I read the linux kernel 2.2.13 source code,and find that in
> > arch/i386/kernel/process.c switch_to function use inline asm 'ltr' to load
> > the new task register which I thought can be loaded automatically when
> > 'iretd','call',or 'ljmp' executed. anyone know why it still use 'ltr' here?
>
> Because TSS gates are _slow_. Doing task switch by hands is much faster.
no, thats not the real reason. It's _not_ faster (definitely not in 2.2),
because all kinds of stuff has to be soft-reloaded and ltr isnt very fast
either. The real reason for doing soft-reloads is threefold:
- exception handling wrt. segment reloading is simpler. It's
conceptually impossible to determine in a simple way what
caused an exception during a TSS switch. It's a 'too complex'
operation to be described with a simple fault. Soft-switching
sorts all the fault sources out nicely.
- the number of TSSs per GDT is limited to 8192, which together
with the LDT entry (and a few other descriptors) constrained the
maximum number of processes/threads to 4000. Since early 2.3 we
have a per-CPU TSS, which is loaded only once at bootup, so
there is no ltr cost and no limitation (apart from RAM) on the
number of processes/threads.
- since late 2.3 we have another feature: soft-TLB reloading,
which optimizes away the TLB-flush of system-threads. TSS
task-switching only knows about the 'dont flush if page tables
are identical between the previous and next task' optimization.
speed is a secondary reason.
-- mingo
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