Re: [RFC/PATCH v2 21/22] KVM: s390: Add KVM HPAGE capability

[Janosch Frank <frankja@linux.vnet.ibm.com>]

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On 20.12.2017 14:02, Cornelia Huck wrote:
> On Wed, 13 Dec 2017 13:53:32 +0100
> Janosch Frank <frankja@linux.vnet.ibm.com> wrote:
> 
>> KVM huge page backing support can not be easily tested under
>> s390. Currently testing is only possible after most of the guest has
>> already been set up.
>>
>> To indicate, that KVM has huge page backing support, we add the
>> KVM_CAP_S390_HPAGE capability. This does not mean, that transparent
>> huge pages are supported.
> 
> Do you expect to use a different cap for non-1MB huge pages? If yes,
> this should probably be mentioned here.

Yes probably KVM_CAP_S390_HPAGE2, but this will not come in the near future.
However this commit message lacks the information, that we only support
1m pages, I'll add that.

1m guest backing pages ought to be enough for everybody (TM) and the
pain to support 2g pages is expected to be a magnitude bigger than the
one for this patchset.

> 
>>
>> Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
>> ---
>>  Documentation/virtual/kvm/api.txt | 10 ++++++++++
>>  arch/s390/kvm/kvm-s390.c          |  1 +
>>  include/uapi/linux/kvm.h          |  1 +
>>  3 files changed, 12 insertions(+)
>>
>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>> index 57d3ee9..a56b0af 100644
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -4369,3 +4369,13 @@ Parameters: none
>>  This capability indicates if the flic device will be able to get/set the
>>  AIS states for migration via the KVM_DEV_FLIC_AISM_ALL attribute and allows
>>  to discover this without having to create a flic device.
>> +
>> +8.14 KVM_CAP_S390_HPAGE
>> +
>> +Architectures: s390
>> +This capability, if KVM_CHECK_EXTENSION indicates that it is
>> +available, means that KVM supports VMs that are memory backed through
>> +hugetlbfs with 1 megabyte pages.
>> +
>> +While it is generally possible to create and start a VM without this
>> +support, the VM will not be functional.
> 
> This sentence applies only to the hugepage case, doesn't it?

Yes, of course.
I'll do a: s/a/such a/

Expect a QEMU fencing patch next year.


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