On Mon, May 09, 2016 at 06:39:26PM +0100, Daniel Thompson wrote:
Current versions of gdb do not interoperate cleanly with kgdb on arm64 systems because gdb and kgdb do not use the same register description. This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1).
Compatibility with gdb (after the patch is applied) is as follows:
gdb-7.6 and earlier Ok gdb-7.7 series Works if user provides custom target description gdb-7.8(.0) Works if user provides custom target description gdb-7.8.1 and later Ok
When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was introduced it was paired with a gdb patch that made an incompatible change to the gdbserver protocol. This patch was eventually merged into the gdb sources: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git%3Ba=commit%3Bh=a4d9...
The change to the protocol was mostly made to simplify big-endian support inside the kernel gdb stub.
While that was how we discovered the inconsistency, a major concern is that SPSR_EL* (i.e. PSTATE), as accessed by MRS/MSR is a 64-bit quantity, even if the upper 32 bits are RES0 today.
It is conceivable that the upper 32 bits could be used in future (as happened with CLIDR_EL1), and for this reason we expose those upper 32 bits from the kernel, and treat system registers as 64-bit quantities generally.
So this was also about ensuring the interface was consistent and to some extent future-proof.
Thanks, Mark.