[PATCH v4 10/15] KVM: arm: implement world switch for debug registers

[christoffer.dall@linaro.org (Christoffer Dall)]

On Tue, Sep 29, 2015 at 01:32:35PM +0800, Zhichao Huang wrote:
> 
> 
> On 2015/9/2 22:53, Christoffer Dall wrote:
> >> +/* Reads cp14 registers from hardware.
> >> + * Writes cp14 registers in-order to the CP14 struct pointed to by r10
> >> + *
> >> + * Assumes vcpu pointer in vcpu reg
> >> + *
> >> + * Clobbers r2-r12
> >> + */
> >> +.macro save_debug_state
> >> +	read_hw_dbg_num
> >> +	cp14_read_and_str r10, 4, cp14_DBGBVR0, r11
> >> +	cp14_read_and_str r10, 5, cp14_DBGBCR0, r11
> >> +	cp14_read_and_str r10, 6, cp14_DBGWVR0, r12
> >> +	cp14_read_and_str r10, 7, cp14_DBGWCR0, r12
> >> +
> >> +	/* DBGDSCR reg */
> >> +	mrc	p14, 0, r2, c0, c1, 0
> >> +	str	r2, [r10, #CP14_OFFSET(cp14_DBGDSCRext)]
> > 
> > so again we're touching the scary register on every world-switch.  Since
> > it sounds like we have experience telling us that this can cause
> > troubles, I'm wondering if we can get around it by:
> > 
> > Only ever allow the guest to use debugging registers if we managed to
> > enter_monitor_mode on the host, and in that case only allow guest
> > debugging with the configuration of DBGDSCR that the host has.
> > 
> > If the host never managed to enable debugging, the guest probably won't
> > succeed either, and we should just trap all guest accesses to the debug
> > registers.
> > 
> > Does this work?
> > 
> 
> I think it works. Since the register is dangerous, we will try not to
> world switch it. It means that the guest will not be able to write the register,
> and will always see what the host set. So the guest will not be able to use
> hardware debug feature if the host disable it.
> 
I talked to Will about this the last day of Linaro Connect and we
arrived at the conclusion that since you cannot trap on only a subset of
the debug registers (namely the DBGDSCR and the registers related to the
OS lock etc.), there is simply no safe and robus way to do this on
ARMv7.

Therfore, this patch series probably needs to be reworked so that we
*always* set TDA for the guest, and when the guest programs a break- or
watchpoint, then we program the hardware registers directly in KVM
without telling perf or hw_breakpoints about this, and we only do any of
this if monitor_mode_enabled() is true.  If not, then debugging inside
the guest simply won't work.

I also don't think that checking the host's breakpoint and wathcpoint
registers are necessary after all, since they are context switched per
process on the host, so this will only be a problem for guests when the
user uses GDB on the VCPU thread (in which case he's asking for trouble
anyway) or with using the weird perf CPU-wide breakpoints, which is very
far from a the common case, so I wouldn't worry about this for now.

Hopefully this makes sense.

Thanks,
-Christoffer