Excerpts from Cédric Le Goater's message of March 23, 2021 2:01 am:
> On 3/22/21 2:15 PM, Nicholas Piggin wrote:
>> Excerpts from Alexey Kardashevskiy's message of March 22, 2021 5:30 pm:
>>>
>>>
>>> On 06/03/2021 02:06, Nicholas Piggin wrote:
>>>> In the interest of minimising the amount of code that is run in>>> "real-mode", don't handle hcalls in real mode in the P9 path.
>>>>
>>>> POWER8 and earlier are much more expensive to exit from HV real mode
>>>> and switch to host mode, because on those processors HV interrupts get
>>>> to the hypervisor with the MMU off, and the other threads in the core
>>>> need to be pulled out of the guest, and SLBs all need to be saved,
>>>> ERATs invalidated, and host SLB reloaded before the MMU is re-enabled
>>>> in host mode. Hash guests also require a lot of hcalls to run. The
>>>> XICS interrupt controller requires hcalls to run.
>>>>
>>>> By contrast, POWER9 has independent thread switching, and in radix mode
>>>> the hypervisor is already in a host virtual memory mode when the HV
>>>> interrupt is taken. Radix + xive guests don't need hcalls to handle
>>>> interrupts or manage translations.
>
> Do we need to handle the host-is-a-P9-without-xive case ?
I'm not sure really. Is there an intention for OPAL to be able to
provide a fallback layer in the worst case? Maybe microwatt grows
HV capability before XIVE?
>
>>>> So it's much less important to handle hcalls in real mode in P9.
>>>
>>> So acde25726bc6034b (which added if(kvm_is_radix(vcpu->kvm))return
>>> H_TOO_HARD) can be reverted, pretty much?
>>
>> Yes. Although that calls attention to the fact I missed doing
>> a P9 h_random handler in this patch. I'll fix that, then I think
>> acde2572 could be reverted entirely.
>>
>> [...]
>>
>>>> } else {
>>>> kvmppc_xive_push_vcpu(vcpu);
>>>> trap = kvmhv_load_hv_regs_and_go(vcpu, time_limit, lpcr);
>>>> - kvmppc_xive_pull_vcpu(vcpu);
>>>> + /* H_CEDE has to be handled now, not later */
>>>> + /* XICS hcalls must be handled before xive is pulled */
>>>> + if (trap == BOOK3S_INTERRUPT_SYSCALL &&
>>>> + !(vcpu->arch.shregs.msr & MSR_PR)) {
>>>> + unsigned long req = kvmppc_get_gpr(vcpu, 3);
>>>>
>>>> + if (req == H_CEDE) {
>>>> + kvmppc_cede(vcpu);
>>>> + kvmppc_xive_cede_vcpu(vcpu); /* may un-cede */
>>>> + kvmppc_set_gpr(vcpu, 3, 0);
>>>> + trap = 0;
>>>> + }
>>>> + if (req == H_EOI || req == H_CPPR ||
>>>
>>> else if (req == H_EOI ... ?
>>
>> Hummm, sure.
>
> you could integrate the H_CEDE in the switch statement below.
Below is in a different file just for the emulation calls.
>>
>> [...]
>>
>>>> +void kvmppc_xive_cede_vcpu(struct kvm_vcpu *vcpu)
>>>> +{
>>>> + void __iomem *esc_vaddr = (void __iomem *)vcpu->arch.xive_esc_vaddr;
>>>> +
>>>> + if (!esc_vaddr)
>>>> + return;
>>>> +
>>>> + /* we are using XIVE with single escalation */
>>>> +
>>>> + if (vcpu->arch.xive_esc_on) {
>>>> + /*
>>>> + * If we still have a pending escalation, abort the cede,
>>>> + * and we must set PQ to 10 rather than 00 so that we don't
>>>> + * potentially end up with two entries for the escalation
>>>> + * interrupt in the XIVE interrupt queue. In that case
>>>> + * we also don't want to set xive_esc_on to 1 here in
>>>> + * case we race with xive_esc_irq().
>>>> + */
>>>> + vcpu->arch.ceded = 0;
>>>> + /*
>>>> + * The escalation interrupts are special as we don't EOI them.
>>>> + * There is no need to use the load-after-store ordering offset
>>>> + * to set PQ to 10 as we won't use StoreEOI.
>>>> + */
>>>> + __raw_readq(esc_vaddr + XIVE_ESB_SET_PQ_10);
>>>> + } else {
>>>> + vcpu->arch.xive_esc_on = true;
>>>> + mb();
>>>> + __raw_readq(esc_vaddr + XIVE_ESB_SET_PQ_00);
>>>> + }
>>>> + mb();
>>>
>>>
>>> Uff. Thanks for cut-n-pasting the comments, helped a lot to match this c
>>> to that asm!
>>
>> Glad it helped.
>>>> +}
>
> I had to do the PowerNV models in QEMU to start understanding that stuff ...
>
>>>> +EXPORT_SYMBOL_GPL(kvmppc_xive_cede_vcpu);
>>>> +
>>>> /*
>>>> * This is a simple trigger for a generic XIVE IRQ. This must
>>>> * only be called for interrupts that support a trigger page
>>>> @@ -2106,6 +2140,32 @@ static int kvmppc_xive_create(struct kvm_device *dev, u32 type)
>>>> return 0;
>>>> }
>>>>
>>>> +int kvmppc_xive_xics_hcall(struct kvm_vcpu *vcpu, u32 req)
>>>> +{
>>>> + struct kvmppc_vcore *vc = vcpu->arch.vcore;
>>>
>>>
>>> Can a XIVE enabled guest issue these hcalls? Don't we want if
>>> (!kvmppc_xics_enabled(vcpu)) and
>>> if (xics_on_xive()) here, as kvmppc_rm_h_xirr() have? Some of these
>>> hcalls do write to XIVE registers but some seem to change
>>> kvmppc_xive_vcpu. Thanks,
>>
>> Yes I think you're right, good catch. I'm not completely sure about all
>> the xive and xics modes but a guest certainly can make any kind of hcall
>> it likes and we have to sanity check it.
>
> Yes.
>
>> We want to take the hcall here (in replacement of the real mode hcalls)
>> with the same condition. So it would be:
>>
>> if (!kvmppc_xics_enabled(vcpu))
>> return H_TOO_HARD;
>
> Yes.
>
> This test covers the case in which a vCPU does XICS hcalls without QEMU
> having connected the vCPU to a XICS ICP. The ICP is the KVM XICS device
> on P8 or XICS-on-XIVE on P9. It catches QEMU errors when the interrupt
> mode is negotiated, we don't want the OS to do XICS hcalls after having
> negotiated the XIVE interrupt mode.
Okay.
> It's different for the XIVE hcalls (when running under XICS) because they
> are all handled in QEMU.
XIVE guest hcalls running on XICS host?
>> if (!xics_on_xive())
>> return H_TOO_HARD;
>
> I understand that this code is only called on P9 and with translation on.
Yes.
> On P9, we could have xics_on_xive() == 0 if XIVE is disabled at compile
> time or with "xive=off" at boot time. But guests should be supported.
> I don't see a reason to restrict the support even if these scenarios
> are rather unusual if not very rare.
>
> on P10, it's the same but since we don't have the XICS emulation layer
> in OPAL, the host will be pretty useless. We don't care.
>
> Since we are trying to handle hcalls, this is L0 and it can not be called
> for nested guests, which would be another case of xics_on_xive() == 0.
> We don't care either.
Okay so no xics_on_xive() test. I'll change that.
Thanks,
Nick
