linux-kernel - Re: [PATCH v3 21/23] KVM: s390: Enable 1M pages for gmap

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Message-ID: <95a76871-771a-4fac-9771-c9e1f4a888ea@de.ibm.com>
Date: Thu, 6 Nov 2025 17:37:57 +0100
From: Christian Borntraeger <borntraeger@...ibm.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, frankja@...ux.ibm.com, nsg@...ux.ibm.com,
        nrb@...ux.ibm.com, seiden@...ux.ibm.com, schlameuss@...ux.ibm.com,
        hca@...ux.ibm.com, svens@...ux.ibm.com, agordeev@...ux.ibm.com,
        gor@...ux.ibm.com, david@...hat.com, gerald.schaefer@...ux.ibm.com
Subject: Re: [PATCH v3 21/23] KVM: s390: Enable 1M pages for gmap



Am 06.11.25 um 17:36 schrieb Claudio Imbrenda:
> On Thu, 6 Nov 2025 17:22:33 +0100
> Christian Borntraeger <borntraeger@...ibm.com> wrote:
> 
>> Am 06.11.25 um 17:11 schrieb Claudio Imbrenda:
>>> While userspace is allowed to have pages of any size, the new gmap
>>> would always use 4k pages to back the guest.
>>>
>>> Enable 1M pages for gmap.
>>>
>>> This allows 1M pages to be used to back a guest when userspace is using
>>> 1M pages for the corresponding addresses (e.g. THP or hugetlbfs).
>>>
>>> Remove the limitation that disallowed having nested guests and
>>> hugepages at the same time.
>>
>> Nice. This might allow us to enable hpage=1 as new default as soon as
>> things stabilize.
>>
>> We would also be able to use 2GB huge pages for the qemu mapping?
> 
> yes, but I don't think that userspace can have 2G THPs right now

Not THP, but hugetlbfs.