Hollis Blanchard wrote: > On Wednesday 14 May 2008 10:28:51 Jan Kiszka wrote: >> So gdb on power relies only on those few hw-breakpoints? With x86 you >> can perfectly run gdb (with soft BPs) in parallel with the gdbstub >> (currently based on hw-BPs, but the same would be true for soft-BPs >> inserted by the gdbstub). > > GDB on Power inserts trap instructions, i.e. standard "soft" breakpoints. It > does not rely on the hardware breakpoints. > > It gets a little more complicated when you involve a GDB stub. IIRC, GDB will > ask the stub to set the breakpoint, and if the stub doesn't support it, GDB > will fall back to overwriting the instructions in memory. Does the Qemu GDB > stub advertise breakpoint support?
Yes, QEMU reacts on both Z0 (soft-BP) and Z1 (hard-BP). That's something even gdbserver does not do! It just handles watchpoints (Z2..4). > > If not, the only support needed in KVM would be to send all debug interrupts > to qemu, and allow qemu to send them back down for in-guest breakpoints. > Simply returning "unsupported" on Z0 is not yet enough for x86, KVM's kernel side should not yet inform QEMU about soft-BP exceptions. But in theory, this should be easily fixable (or is already the case for other archs). And it would safe us from keeping track of N software breakpoints, where N could even become larger than 32, the current hardcoded limit for plain QEMU. :) Meanwhile I realized that the proposed KVM_DEBUG_GUEST API is insufficient: We need a return channel for the debug register state (specifically to figure out details about hit watchpoints). I'm now favoring KVM_SET_DEBUG and KVM_GET_DEBUG as two new IOCTLs, enabling us to write _and_ read-back the suggested data structure. Jan
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