Re: [PATCH v2 13/25] KVM: arm64: vgic-v3: Add ICV_BPR0_EL1 handler

Marc Zyngier
2017-06-06T16:56:54Z

On 06/06/17 16:56, Peter Maydell wrote:
> On 6 June 2017 at 16:46, Christoffer Dall <cdall@linaro.org> wrote:
>> On Tue, Jun 06, 2017 at 04:15:05PM +0100, Marc Zyngier wrote:
>>>>> +static void __hyp_text __vgic_v3_write_bpr0(struct kvm_vcpu *vcpu, u32 vmcr, int rt)
>>>>> +{
>>>>> + u64 val = vcpu_get_reg(vcpu, rt);
>>>>> + u8 bpr_min = 7 - vtr_to_nr_pre_bits(read_gicreg(ICH_VTR_EL2));
>>>>> +
>>>>> + /* Enforce BPR limiting */
>>>>> + if (val < bpr_min)
>>>>> + val = bpr_min;
>>>>> +
>>>>> + val <<= ICH_VMCR_BPR0_SHIFT;
>>>>> + val &= ICH_VMCR_BPR0_MASK;
>>>>> + vmcr &= ~ICH_VMCR_BPR0_MASK;
>>>>> + vmcr |= val;
>>>>> +
>>>>> + if (vmcr & ICH_VMCR_CBPR_MASK) {
>>>>> + val = __vgic_v3_get_bpr1(vmcr);
>>>>> + val <<= ICH_VMCR_BPR1_SHIFT;
>>>>> + val &= ICH_VMCR_BPR1_MASK;
>>>>> + vmcr &= ~ICH_VMCR_BPR1_MASK;
>>>>> + vmcr |= val;
>>>>> + }
>>>>
>>>> I don't understand why this block is needed?
>>>
>>> If you have CBPR already set, and then update BPR0, you need to make
>>> sure that BPR1 gets updated as well. You could hope that the HW would do
>>> it for you, but since we're erratum workaround land...
>>>
>>
>> I just didn't read the spec that way, I gathered that the hardware would
>> maintain read-as-written for for bpr1 but use bpr0 to set the binary
>> point when cbpr is set, and just ignore writes to bpr1 for as long as
>> cbpr is set.
>
> This depends whether you're talking about the ICC/ICV BPR0/BPR1 registers,
> or the fields in the ICH_VMCR*. For the former, if CBPR is set then
> BPR1 reads and writes affect BPR0. For the latter, the two fields
> are both independent and read-as-written regardless of the value
> of CBPR, it's just that the value in the BPR1 field has no effect.
> (The reason for this is for VM state migration: if a guest does:
> - write X to BPR0
> - write Y to BPR1
> - set CBPR
> - write Z to BPR0
> - unset CBPR
> - read BPR1
> it should get back Y still; so EL2 needs to have a way to see
> both the underlying BPR0 and BPR1 values even if CPBR is in effect.)
>
> So the code above is not correct, I think, because it's making
> writes to BPR0 affect the BPR1 value. Instead what should happen
> is that when we emulate reads and writes to ICC_BPR1 we should pay
> attention to CBPR and work with either the VMCR BPR0 or BPR1 field
> appropriately.

The BPR1 accessors are already behaving that way (write ignored and read
as BPR0+1 if CBPR is set). But I didn't realize that you could mess with
CBPR and BPR0, and yet have ICH_VMCR_EL2.VBPR1 be unaffected.

I'll drop that hunk.

Thanks,

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