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

[Zhichao Huang <zhichao.huang@linaro.org>]

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.

> Thanks,
> -Christoffer
> 
>>  	.if \operation == vmentry
>>  	orr	r2, r2, r3		@ Trap some perfmon accesses
>>  	.else
>> -- 
>> 1.7.12.4
>>