Re: [Xen-devel] [PATCH 7/7] arch.monitor: move bits to common (arch_domain to domain)

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

On 2/8/2016 8:29 PM, Tamas K Lengyel wrote:

On Mon, Feb 8, 2016 at 9:58 AM, Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx> wrote:

This patch moves bitfield members for single-step, software-breakpoint and
guest-request monitor vm-events from the arch-side (struct arch_domain) to
the common-side (struct domain). Ctrl-reg bits (i.e. write_ctrlreg_* members)
are left on the arch-side, because control-registers number can vary across
architectures.

Signed-off-by: Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>

Technically this looks fine, but I do wonder if and what plans you have to actually implement these events for ARM.

Currently I've only planned implementations for control-register write events & guest-requests.
The other two also seem feasible though, I might give adding those a shot sometime after sending the other patches.

I haven't spent too much time looking into it, but I'm not aware of equivalent features on ARM to Intel MTF (singlestepping) or to software-breakpoint trapping. The only instruction I know that functionally comes close to software-breakpoint trapping (INT3) is the SMC instruction which can be trapped into the VMM, but I would not call that a "breakpoint" in the traditional sense.

Tamas

There's the debugging architecture, hypervisor control of that is possible on both 32-bit & 64-bit ARM.
It isn't as easy as for X86 though, where MTF is a hypervisor-internal feature and INT3 can be
trapped specifically, whereas on ARM granularity of trap-setting is less of a concern apparently.
For this reason, the only issue I see here is the performance penalty these traps would cause for
arbitrary software breakpoints (for obvious reasons that doesn't matter in the case of single-stepping).

For INT3, the ARM equivalent is be the BKPT/BRK (set HDCR.TDE on AArch32/MDCR_EL2.TDE AArch64) instruction.
Trapping on this instruction implies trapping on
- AArch32: some other debug exceptions (looking @ B1.8.9, ARMv7 DDI 0406C.b)
- AArch64: *all software debug exceptions* + *all debug register accesses* (this might cause some headaches)
For MTF-like functionality, the debug architecture also provides ways for single-stepping.
That would similarly generate software breakpoint exceptions which can be routed to the hypervisor.

Corneliu.

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