linux-kernel - Re: [PATCH v3 00/23] KVM: s390: gmap rewrite, the real deal

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Message-ID: <20251118152815.9674D33-hca@linux.ibm.com>
Date: Tue, 18 Nov 2025 16:28:15 +0100
From: Heiko Carstens <hca@...ux.ibm.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, nsg@...ux.ibm.com, nrb@...ux.ibm.com,
        seiden@...ux.ibm.com, schlameuss@...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 00/23] KVM: s390: gmap rewrite, the real deal

On Thu, Nov 06, 2025 at 05:10:54PM +0100, Claudio Imbrenda wrote:
> This series is the last big series of the gmap rewrite. It introduces
> the new code and actually uses it. The old code is then removed.
> 
> The insertions/deletions balance is negative both for this series, and
> for the whole rewrite, also considering all the preparatory patches.
> 
> KVM on s390 will now use the mmu_notifier, like most other
> architectures. The gmap address space is now completely separate from
> userspace; no level of the page tables is shared between guest mapping
> and userspace.
> 
> One of the biggest advantages is that the page size of userspace is
> completely independent of the page size used by the guest. Userspace
> can mix normal pages, THPs, hugetlbfs, and more.
> 
> Patches 1 to 6 are mostly preparations; introducing some new bits and
> functions, and moving code around.
> 
> Patch 7 to 16 is the meat of the new gmap code; page table management
> functions and gmap management. This is the code that will be used to
> manage guest memory.
> 
> Patch 18 is unfortunately big; the existing code is converted to use
> the new gmap and all references to the old gmap are removed. This needs
> to be done all at once, unfortunately, hence the size of the patch.
> 
> Patch 19 and 20 remove all the now unused code.
> 
> Patch 21 and 22 allow for 1M pages to be used to back guests, and add
> some more functions that are useful for testing.
> 
> Patch 23 fixes storage key manipulation functions, which would
> otherwise be broken by the new code.

I would guess patch 23 also needs to go into the already huge patch which
switches everything to the new gmap code, since otherwise bisect will not work
for anything that is storage key related.

Anyway, I can imagine some addon cleanups, but that can wait after this series
is upstream.

At least from a "core s390 code view", without considering kvm this looks good
to me. There is at least one known bug hiding in this huge rewrite - but just
wanted to let you know that my concerns with the previous version have been
addressed.