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  • Bug#1106757: ppc64el Trixie: Need default 64KB page kernel or installer

    From Cyril Brulebois@21:1/5 to All on Thu May 29 16:00:01 2025
    XPost: linux.debian.kernel, linux.debian.maint.boot

    Hi,

    Looping in the kernel team.

    Trupti <trupti@linux.ibm.com> (2025-05-29):
    Testing with the debian-testing-ppc64el-DVD-1.iso confirms that Debian
    Trixie on ppc64el currently installs a 4KB page kernel by default. This default (ref: #1074217) causes issues, especially for KVM virtualization (e.g., "Can't support 64 kiB guest pages with 4 kiB host pages"), hindering common ppc64el use cases. Debian Trixie should offer easier access to a 64KB page size kernel.

    While 64KB kernel variants, such as those provided by the linux-image-powerpc64le-64k meta-package, are available in the Trixie archive, their manual installation post-setup is inconvenient.

    Request:

    Make a 64KB page kernel the default for ppc64el Trixie.

    My reading of debian/config/ppc64el/defines.toml in kernel-team/linux.git suggests they are responsible for setting up the default, not d-i.

    OR

    Add a clear installer option to select a 64KB page kernel.

    I believe this should be offered in expert mode, have you tried that?

    This would improve usability for ppc64el users needing 64KB pages for performance or virtualization.


    Cheers,
    --
    Cyril Brulebois (kibi@debian.org) <https://debamax.com/>
    D-I release manager -- Release team member -- Freelance Consultant

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  • From Pascal Hambourg@21:1/5 to Cyril Brulebois on Thu May 29 21:40:01 2025
    XPost: linux.debian.maint.boot

    (Removing kernel team)

    On 29/05/2025 at 15:54, Cyril Brulebois wrote:
    Trupti <trupti@linux.ibm.com> (2025-05-29):

    Add a clear installer option to select a 64KB page kernel.

    I believe this should be offered in expert mode, have you tried that?

    AFAICS linux-image-powerpc64le-64k is not included in the DVD-1 image,
    so it requires a network mirror. Same with linux-image-arm64-16k.

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  • From Bastian Blank@21:1/5 to Trupti on Wed Jun 4 23:10:01 2025
    XPost: linux.debian.maint.boot

    On Thu, May 29, 2025 at 04:04:43PM +0530, Trupti wrote:
    Testing with the debian-testing-ppc64el-DVD-1.iso confirms that Debian
    Trixie on ppc64el currently installs a 4KB page kernel by default. This default (ref: #1074217) causes issues, especially for KVM virtualization (e.g., "Can't support 64 kiB guest pages with 4 kiB host pages"), hindering common ppc64el use cases. Debian Trixie should offer easier access to a 64KB page size kernel.

    This is a bug in qemu.

    Make a 64KB page kernel the default for ppc64el Trixie.

    We decided to make 4 KiB the default for all architectures.

    Bastian

    --
    Vulcans never bluff.
    -- Spock, "The Doomsday Machine", stardate 4202.1

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  • From Bastian Blank@21:1/5 to Bastian Blank on Thu Jun 5 12:10:01 2025
    XPost: linux.debian.maint.boot

    On Wed, Jun 04, 2025 at 10:32:13PM +0200, Bastian Blank wrote:
    On Thu, May 29, 2025 at 04:04:43PM +0530, Trupti wrote:
    Testing with the debian-testing-ppc64el-DVD-1.iso confirms that Debian Trixie on ppc64el currently installs a 4KB page kernel by default. This default (ref: #1074217) causes issues, especially for KVM virtualization (e.g., "Can't support 64 kiB guest pages with 4 kiB host pages"), hindering common ppc64el use cases. Debian Trixie should offer easier access to a 64KB
    page size kernel.

    This is a bug in qemu.

    Could you please contact the qemu maintainer via
    1107288@bugs.debian.org? I can only do theoretical work on a fix for
    qemu, but you might have access to the hardware as well.

    Bastian

    --
    Witch! Witch! They'll burn ya!
    -- Hag, "Tomorrow is Yesterday", stardate unknown

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  • From Bastian Blank@21:1/5 to Michael Tokarev on Thu Jun 5 15:20:01 2025
    XPost: linux.debian.maint.boot

    On Thu, Jun 05, 2025 at 02:13:59PM +0300, Michael Tokarev wrote:
    But I know right to nothing about ppc64 and its page sizes.

    Okay, so we should remove kvm support from ppc64*, because it is not
    supported anymore?

    Bastian

    --
    Insults are effective only where emotion is present.
    -- Spock, "Who Mourns for Adonais?" stardate 3468.1

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  • From Bastian Blank@21:1/5 to Michael Tokarev on Thu Jun 5 17:50:02 2025
    XPost: linux.debian.maint.boot

    On Thu, Jun 05, 2025 at 02:13:59PM +0300, Michael Tokarev wrote:
    I definitely don't want to change upstream defaults here.

    And noone asked for changed defaults, just overriding them when it
    already saw it will not work.

    Bastian

    --
    A princess should not be afraid -- not with a brave knight to protect her.
    -- McCoy, "Shore Leave", stardate 3025.3

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  • From Lennart Sorensen@21:1/5 to Bastian Blank on Thu Jun 5 19:00:01 2025
    XPost: linux.debian.maint.boot

    On Thu, Jun 05, 2025 at 03:15:18PM +0200, Bastian Blank wrote:
    Okay, so we should remove kvm support from ppc64*, because it is not supported anymore?

    It looked to me as if all you have to do is pass a command line argument
    to qemu-kvm telling it to use 4k page size instead of the default 64k.
    Doesn't mean it doesn't support kvm.

    Changing the default sounds like it could break kvm migration between
    hosts. Perhaps it would be friendly if qemu would detect the page size
    doesn't work and print a message telling the user what argument they
    have to add to work with 4k page systems.

    --
    Len Sorensen

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  • From Pascal Hambourg@21:1/5 to Pascal Hambourg on Thu Jun 12 23:10:01 2025
    XPost: linux.debian.maint.boot

    On 29/05/2025 at 21:19, Pascal Hambourg wrote:
    On 29/05/2025 at 15:54, Cyril Brulebois wrote:
    Trupti <trupti@linux.ibm.com> (2025-05-29):

    Add a clear installer option to select a 64KB page kernel.

    I believe this should be offered in expert mode, have you tried that?

    AFAICS linux-image-powerpc64le-64k is not included in the DVD-1 image,
    so it requires a network mirror. Same with linux-image-arm64-16k.

    Thinking twice, DVD-1 and netinst images set up a network mirror only
    after installing the base system, so they only offer kernel flavours
    included in installation images. Only netboot images set up a network
    mirror before installing the base system and offer all kernel flavours.

    After reading #1079800 ("release-notes - Document that default ppc64el
    kernel switched to 4k pages") [1], I thought about possible impact of
    page size change on the installer, but do not have any ppc64 or arm64
    hardware to test.

    [1] <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1079800>

    The kernel supports a swap space only if it matches the page size.
    The kernel supports a btrfs filesystem with a sector size equal to the
    page size or 4k (aka subpage block size support, see [2][3]).

    [2] <https://btrfs.readthedocs.io/en/latest/Subpage.html>
    [3] <https://btrfs.readthedocs.io/en/latest/Status.html#status-subpage-block-size>

    Bookworm:
    - linux-image-powerpc64le has 64k page size.
    - mkswap creates swap with 64k page size.
    - mkfs.btrfs creates filesystem with 64k sector size.

    Trixie:
    - linux-image-powerpc64le has 4k page size.
    - mkswap creates swap with 4k page size.
    - mkfs.btrfs creates filesystem with 4k sector size.

    1) Impact on reuse of existing swap and btrfs filesystem:

    The installer can detect (via libparted) and use an existing swap only
    if it matches the installer kernel page size (4k). So it cannot use a
    swap space created by a previous version with 64k page size without reformatting it. No big deal though.

    The installer can detect (via libparted) an existing btrfs filesystem regardless of its sector size but can use it only if its sector size is
    4k. So it cannot use a btrfs filesystem created by a previous version
    with 64k sector size without reformatting it (and losing all its data).

    2) Impact if linux-image-powerpc64le-64k (or linux-image-arm64-16k on
    arm64) is selected:

    systemd.swap seems to run swapon with --fixpgsz which reinitializes the
    swap space if it does not match the kernel page size, so swap page size mismatch should be fixed automatically during first boot.

    A kernel with 64k page size is said to support btrfs filesystems with
    either 4k or 64k sector size, so it should be able to mount btrfs
    filesystems with 4k sector size created by Trixie installer.

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