[PATCH 09/12] KVM: PPC: Book3S HV nestedv2: Do not call H_COPY_TOFROM_GUEST

Vaibhav Jain vaibhav at linux.ibm.com
Mon Dec 18 18:02:39 AEDT 2023

Hi Aneesh,

"Aneesh Kumar K.V" <aneesh.kumar at kernel.org> writes:

>> Yes, Agreed and thats a nice suggestion. However ATM the hypervisor 
>> supporting Nestedv2 doesnt have support for this hcall. In future
>> once we have support for this hcall for nestedv2 from the hypervisor
>> we can replace this branch with a firmware_has_feature() test.
> What I am suggesting is we convert that conditional to firmware_has_feature so that
> later when hypervisor supports this hcall all older kernel can make
> use of the copy_tofrom_guest without any code change.

AFAIK for firmware_has_feature to work we either need:
- A way to call this hcall with some invalid args. However lpid/pid for
guest arent allocated during boot.

- A way for hypervisor to advertise support for this hcall before the L1
kernel boots.

ATM L0 dosent support for any of these two ways. I can do a follow up
patch later when we have a clarity on how we want to advertise support
for this hcall. For now current kernel supporting nestedv2 wont be
using this hcall assuming its not supported. Future kernels can use one
of the two ways above to set the firmware_has_feature flag to take
advantage of this hcall.

~ Vaibhav

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