Re: [PATCH v2 9/9] arm64: KVM: enable trapping of all debug registers

Marc Zyngier
2014-05-28T16:10:10Z

On 25/05/14 16:36, Christoffer Dall wrote:
> On Tue, May 20, 2014 at 05:55:45PM +0100, Marc Zyngier wrote:
>> Enable trapping of the debug registers, preventing the guests to
>> mess with the host state (and allowing guests to use the debug
>> infrastructure as well).
>>
>> Reviewed-by: Anup Patel <anup.patel@linaro.org>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>> arch/arm64/kvm/hyp.S | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>> index 73ec5c4..72ed0bf8 100644
>> --- a/arch/arm64/kvm/hyp.S
>> +++ b/arch/arm64/kvm/hyp.S
>> @@ -777,6 +777,14 @@ __kvm_hyp_code_start:
>> mrs x2, mdcr_el2
>> and x2, x2, #MDCR_EL2_HPMN_MASK
>> orr x2, x2, #(MDCR_EL2_TPM | MDCR_EL2_TPMCR)
>> + orr x2, x2, #(MDCR_EL2_TDRA | MDCR_EL2_TDOSA)
>
> so we unconditionally trap on the OS register access, but we don't
> properly emulate these do we? What's the rationale? (atmittedly,
> again, I'm not 100% clear on how the OS lock thingy is supposed to
> work/be used).

The rational is that we don't want the guest to mess with the host
state, which may have decided to use the OSlock thing (we don't use it
at all, but who knows...). So we trap it, discard whatever the guest
wants to put there, and carry on.

I'm not sure if this would confuse any guest (we only have Linux so far,
so I'm not too worried). Should a more adventurous guest show up, we can
revisit this.

M.
--
Jazz is not dead. It just smells funny...