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

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Message-ID: <20251106173650.31907261@p-imbrenda>
Date: Thu, 6 Nov 2025 17:36:50 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: Christian Borntraeger <borntraeger@...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

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

> With this patch we then have 1MB pages for the guest mapping.
> 
> As a future improvement, maybe also allow guest mapping 2GB pages.

the core of the new gmap code introduced in this series supports 2G
pages already; to enable 2G pages, a simple patch similar to this one
is all that's needed.

I refrained from enabling 2G pages in this series because this series
is already too big, and also I would not be able to test it easily.