[PATCH v4 22/46] KVM: PPC: Book3S HV P9: Stop handling hcalls in real-mode in the P9 path

[Nicholas Piggin]

Excerpts from Fabiano Rosas's message of March 24, 2021 4:03 am:
> Nicholas Piggin <npiggin at gmail.com> writes:
> 
>> 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.
>>
>> So it's much less important to handle hcalls in real mode in P9.
>>
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> 
> I tried this again in the L2 with xive=off and it works as expected now.
> 
> Tested-by: Fabiano Rosas <farosas at linux.ibm.com>

Oh good, thanks for spotting the problem and re testing.

Thanks,
Nick