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Description
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Currently the sparc brandz syscall wrappers are only compiled and active
on DEBUG kernels. this was done so that there would be no performance
impact on native processes on production systems, but we could still
test the brandz framework on sparc systems by using DEBUG kernels.
unfortunatly the lack of support for brandz callback interposition
on non-DEBUG kernels prevents the introduction of real sparc brands.
there for the current syscall interposition wrappers should be compiled
for both DEBUG and non-DEBUG kernels, and the performance impact of
enabling this should be quantified and minimized.
some ideas for improving the performance of the current syscall interposition
mechanism (which involves checking if a processes is branded in the
syscall trap handler) would be to introduce a branded bit into either the
cpu structure or the thread structure. (currently we need to access the
proc structure to determine if a proceses is branded.) this would reduce
the number of memory accesses required in the trap code to determine if
a process is branded.
another idea would be to have the interposition code compiled in but
disabled by default on systems that don't have any branded zones
running, and then if the administrator attempts to enable a branded
zone we could hot patch the trap table to enable the syscall interposition
mechanism. this approach would insure a zero performance impact on systems
that don't have branded zones enabled.
the fix that was chosen for this bug was to implement hot patching of
the trap table to enable brandz syscall interposition.
basically, now the brandz syscall wrappers will always be compiled
on both DEBUG and non-DEBUG kernels, but by default they will never
be executed. then when the first branded zone is booted on the system,
the zones/brandz framework will hot patch the first instruction of
both the 32 and 64 bit syscall trap entry points. that first instruction
will be replaced with a "ba,a" instruction which will redirect the
all the syscall traps to the brandz syscall wrapper entry points.
when the last branded zone on a system is halted, the original
instructions that were overwritten in the trap table will be
restored, there by disabling the brandz syscall interposition.
this approach should result in a zero performance impact on systems
that don't use branded zones.
this approach was taken after studying how trapstat and the dtrace
lockstat providers work. the technique was carefully chosen to not
cause any problems with trapstat. the new syscall instruction patch
points use a similar naming structure to those found in the lockstat
provider. and the patch points themselves were chosed to not interfere
with trapstat or it's any of it's alternate trap tables. lastly the
hot patching doesn't actually involve introducing any new trap tables
or changing %TBA, it only involves hot patching two instructions in
the existing default system trap table.
the changes in this bug are dependant upon the following changes:
6539890 sparc brandz syscall wrappers don't support sun4v
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