Re: [PATCH v4 15/21] KVM: arm64: Set an impdef ESR for Virtual-SError using VSESR_EL2.

James Morse
2017-10-30T15:44:31Z

Hi Christoffer,

On 30/10/17 10:51, Christoffer Dall wrote:
> On Mon, Oct 30, 2017 at 08:59:51AM +0100, Christoffer Dall wrote:
>> On Thu, Oct 19, 2017 at 03:58:01PM +0100, James Morse wrote:
>>> Prior to v8.2's RAS Extensions, the HCR_EL2.VSE 'virtual SError' feature
>>> generated an SError with an implementation defined ESR_EL1.ISS, because we
>>> had no mechanism to specify the ESR value.
>>>
>>> On Juno this generates an all-zero ESR, the most significant bit 'ISV'
>>> is clear indicating the remainder of the ISS field is invalid.
>>>
>>> With the RAS Extensions we have a mechanism to specify this value, and the
>>> most significant bit has a new meaning: 'IDS - Implementation Defined
>>> Syndrome'. An all-zero SError ESR now means: 'RAS error: Uncategorized'
>>> instead of 'no valid ISS'.
>>>
>>> Add KVM support for the VSESR_EL2 register to specify an ESR value when
>>> HCR_EL2.VSE generates a virtual SError. Change kvm_inject_vabt() to
>>> specify an implementation-defined value.
>>>
>>> We only need to restore the VSESR_EL2 value when HCR_EL2.VSE is set, KVM
>>> save/restores this bit during __deactivate_traps() and hardware clears the
>>> bit once the guest has consumed the virtual-SError.
>>>
>>> Future patches may add an API (or KVM CAP) to pend a virtual SError with
>>> a specified ESR.

>>> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
>>> index 945e79c641c4..af37658223a0 100644
>>> --- a/arch/arm64/kvm/hyp/switch.c
>>> +++ b/arch/arm64/kvm/hyp/switch.c
>>> @@ -86,6 +86,10 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
>>> isb();
>>> }
>>> write_sysreg(val, hcr_el2);
>>> +
>>> + if (cpus_have_const_cap(ARM64_HAS_RAS_EXTN) && (val & HCR_VSE))
>>> + write_sysreg_s(vcpu->arch.vsesr_el2, SYS_VSESR_EL2);
>>> +

>> Just a heads up: If my optimization work gets merged, that will
>> eventually move stuff like this in to load/put hooks for system
>> registers, but I can deal with this easily, also adding a direct write
>> in pend_guest_serror when moving the logic around.

Sure. This would always be called when the vcpu is loaded, so yes it should end
up as a direct write to the system register.

>> However, if we start architecting something more complex, it would be
>> good to keep in mind how to maintain minimum work on the switching path
>> after we've optimized the hypervisor.

I think gengdongjiu's trick of only restoring VSESR if HCR_EL2.VSE is set is the
best we can do here. (Hence the Celebrate-Contribution tag).

For VDISR_EL2 we can probably only save/restore it if its non-zero. On most
systems it will never be touched so the cost is testing that whenever we exit
the guest/unload the vcpu.

> Actually, after thinking about this, if the guest can only see this via
> the ESR if we set the HCR_EL2.VSE, wouldn't it make sense to just set
> this value in pend_guest_serror, and if we're on a non-VHE system --
> assuming that's something we want to support with this 8.2 feature
> -- we jump to EL2 and back to set the value?

It thought this was the 'eventually ... direct write' above.
Once your load/put hooks are merged? Yes, just write it straight to the CPU
register and set the guests HCR_EL2.VSE.

Now? Wouldn't this get lost if we reschedule onto another cpu...

Thanks,

James