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d8f648486e
Fixes BZ#374719 Patch by: klemens <ka7@la-evento.com> git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16336
91 lines
4.5 KiB
Plaintext
91 lines
4.5 KiB
Plaintext
(for general information
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about gdbserver, read coregrind/m_gdbserver/README_DEVELOPERS
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about gdbserver tests, read gdbserver_tests/README_DEVELOPERS).
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ARM thumb and gdbserver
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-----------------------
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When executing thumb code, the ARM cpu program counter value is
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not equal to the address of the instruction being executed: the fact
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that the instruction being executed is a thumb instruction is
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encoded in the bit0 of the program counter.
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For more information about this, see VEX/pub/libvex_guest_arm.h.
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This additional bit set in the ARM guest_IP caused some difficulties
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at several places in the gdbserver code.
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* breakpoint handling:
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gdbsrv code detects that a breakpoint is reached
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by comparing the current guest_IP with the breakpoint addresses given
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by gdb. As gdb is giving breakpoints without the thumb bit set,
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the gdbsrv code has to ensure breakpoint comparisons are done without
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the thumb bit.
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=> the breakpoints are stored in the gdbserver breakpoints hash table
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without the thumb bit.
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=> when the guest_IP is compared with these breakpoints, the thumb bit
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must be masked.
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* Thumb bit in IstMark:
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When instrumenting a SB for gdbserver, a PUT of the current instruction
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address is done to the IP, to ensure that the gdbserver helper call
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sees the correct IP. This PUT must be done with the thumb bit set,
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as this value will be given back to gdb. gdb uses the thumb bit a.o.
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to guess the next program counter after this instruction.
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Such a guess is needed for gdb commands such as next/step/...
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If the thumb bit is not properly set, these commands are not working.
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As gdbserver instrumentation code uses the IstMark to find the IP
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of the instruction being instrumented, the thumb bit must be accessible
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or found. Multiple approaches were tried (see below) before the
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current one (which is to have a field Delta in IstMark which encodes
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the thumb bit).
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* Thumb bit in extents
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When a breakpoint is placed at an instruction, the translation for this
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instruction must be discarded to have the SB re-instrumented for gdbserver.
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At least at this moment (r2155/r11786), the extents for ARM/thumb blocks
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contains addresses with the thumb bit set. It is not clear that this is
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a good thing: extents should preferably reference the real address range of
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instructions, not an address range which can be "off by one" due to the
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fact that the instruction is a thumb instruction.
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Due to this "off by one", gdbserver is discarding a range of two bytes
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to ensure it is not sensitive to the presence (or not) of the thumb
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bit in the range being discarded.
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* Handling of monitor vg.translate
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When the translation is requested, the thumb bit must be properly set
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in the address, as this bit is used by the Valgrind translator
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to decide to decode in ARM or in thumb mode.
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The monitor command guesses the thumb bit based on the debug information.
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* gdb 7.0 is buggy with thumb code
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The logic of gdb to guess the next PC in thumb code is not working
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properly in version gdb 7.0. Several gdbserver tests are disabled
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on ARM if gdb version is < 7.1.
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* The (historical) list of difficulties encountered is the following:
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- initially, IstMark contained the thumb bit in the addr.
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This caused the breakpoints to not work (comparison between
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gdb address and gdbserver failed).
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(see breakpoint handling above).
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- A change was done to mask the thumb bit in the IstMark
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This made breakpoints work, but then gdb next/step/... was then failing
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(see Thumb bit in IstMark above).
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- So, it was needed to have the thumb bit when instrumenting an
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IstMark. 3 solutions were looked at:
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* use the debug info : this solution was discarded as often debug
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info does not allow a 100% correct solution. debug info is acceptable
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for vg.translate (which is only for internal valgrind debugging),
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but is better not used for 'end user functionality'.
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* Derive the thumb bit for a SB from the extent address.
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This was discarded as this implies that an SB cannot mix thumb
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and ARM code. This implied to disable chasing at transition between
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ARM/thumb code, which potentially decreases performance.
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Also, it would oblige to keep the thumb bit in the extents, which
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seems not nice.
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* the final solution implemented was to add a Delta fied in IstMark.
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This Delta field gives the delta to add to the IstMark addr field
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to obtain the guest_IP : for thumb code, this field is 1,
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and is 0 for ARM code and for all other architectures.
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