Re: [PATCH v4 15/15] KVM: arm: enable trapping of all debug registers

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Sep 29, 2015 at 01:41:45PM +0800, Zhichao Huang wrote:
> 
> 
> On 2015/9/3 0:08, Christoffer Dall wrote:
> > On Mon, Aug 10, 2015 at 09:26:07PM +0800, Zhichao Huang wrote:
> >> Enable trapping of the debug registers unconditionally, allowing guests to
> >> use the debug infrastructure.
> >>
> >> Signed-off-by: Zhichao Huang <zhichao.huang@linaro.org>
> >> Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
> >> ---
> >>  arch/arm/kvm/interrupts_head.S | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm/kvm/interrupts_head.S b/arch/arm/kvm/interrupts_head.S
> >> index 7ad0adf..494991d 100644
> >> --- a/arch/arm/kvm/interrupts_head.S
> >> +++ b/arch/arm/kvm/interrupts_head.S
> >> @@ -792,7 +792,7 @@ ARM_BE8(rev	r6, r6  )
> >>   * (hardware reset value is 0) */
> >>  .macro set_hdcr operation
> >>  	mrc	p15, 4, r2, c1, c1, 1
> >> -	ldr	r3, =(HDCR_TPM|HDCR_TPMCR)
> >> +	ldr	r3, =(HDCR_TPM|HDCR_TPMCR|HDCR_TDRA|HDCR_TDOSA|HDCR_TDA)
> > 
> > eh, but I thought we didn't have to trap accesses to the debug registers
> > if we switch them, because the guest is likely to be using them?
> > 
> > Maybe I am getting confused, can you repeat for me exactly when we
> > context-switch the registers and when we trap accesses to them?
> > 
> 
> Since we don't want to world switch the dangerous register(DBGDSCR), we have
> to trap accesses all the time, to prevent the guest to write to the real register.
> 
ok, so this is in line with my comment to your previous patch, but you
did have world-switching code of DBGDSCR in this series, hence my
confusion.  So you would need to get rid of this for the new version of
the series.

Thanks,
-Christoffer