Re: [RFC PATCH v2 38/38] KVM: arm64: Respect the virtual CPTR_EL2.TCPAC setting

[Jintack Lim]

Hi Christoffer,

On Mon, Jul 31, 2017 at 8:59 AM, Christoffer Dall <cdall@xxxxxxxxxx> wrote:
> On Tue, Jul 18, 2017 at 11:59:04AM -0500, Jintack Lim wrote:
>> Forward CPACR_EL1 traps to the virtual EL2 if virtual CPTR_EL2 is
>> configured to trap CPACR_EL1 accesses from EL1.
>>
>> This is for recursive nested virtualization.
>>
>> Signed-off-by: Jintack Lim <jintack.lim@xxxxxxxxxx>
>> ---
>>  arch/arm64/kvm/sys_regs.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 6f67666..ba2966d 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -1091,6 +1091,11 @@ static bool access_cpacr(struct kvm_vcpu *vcpu,
>>       if (el12_reg(p) && forward_nv_traps(vcpu))
>>               return kvm_inject_nested_sync(vcpu, kvm_vcpu_get_hsr(vcpu));
>>
>> +     /* Forward this trap to the virtual EL2 if CPTR_EL2.TCPAC is set*/
>> +     if (!el12_reg(p) && !vcpu_mode_el2(vcpu) &&
>> +         (vcpu_sys_reg(vcpu, CPTR_EL2) & CPTR_EL2_TCPAC))
>> +             return kvm_inject_nested_sync(vcpu, kvm_vcpu_get_hsr(vcpu));
>> +
>
> I'm trying to understand what should happen if the VM is in EL1 and
> accesses CPACR_EL12, but the guest hypervisor did not set
> CPTR_EL2.TCPAC, why would we get here, and if there's a good reason why

I guess what you meant is HCR_EL2.NV bit?

> we god here, is the EL12 access not supposed to undef at EL1 as opposed
> to actually work, like it seems your code does when it doesn't take the
> branch?

IIUC, we need to have this logic

if (el12_reg() && virtual HCR_EL2.NV == 0)
   inject_undef();

This is a good point, and should be applied for all traps controlled by NV bit.

>
>>       /*
>>        * When the virtual HCR_EL2.E2H == 1, an access to CPACR_EL1
>>        * in the virtual EL2 is to access CPTR_EL2.
>> --
>> 1.9.1
>>
>
> Thanks,
> -Christoffer