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

Christoffer Dall
2017-10-31T05:47:17Z

On Mon, Oct 30, 2017 at 03:44:17PM +0000, James Morse wrote:
> 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).

yes, I agree.

>
> 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.
>

I think VDISR_EL2 should just be saved/restored in vcpu_put/load after
the optimization.

>
> > 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.

Yes, that's what I mean.

> 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...
>
>

That's why we'd also save/restore it in vcpu_put/vcpu_load.

So, for VSESR, we'd save/restore it in put/load (conditionally on VSE
being set if we like), and we'd also set it from pend_guest_serror.

For VDISR, it's just saved/restored in put/load.

Thanks,
-Christoffer