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Bug#887061: linux-image-4.9.0-5-amd64: KPTI (kaiser.c): please, no silent disable for Xen guests

Cross-posted to 2 groups.

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Package: src:linux
Version: 4.9.65-3+deb9u2
Severity: wishlist

Hi,

My VM is a Xen guest, and I was confused not to find any information
about KPTI in the kernel logs. This is due to the silent_disable
case here:

In arch/x86/mm/kaiser.c:

void __init kaiser_check_boottime_disable(void)
{
[...]
        if (boot_cpu_has(X86_FEATURE_XENPV))
                goto silent_disable;
[...]
disable:
        pr_info("disabled\n");

silent_disable:
        kaiser_enabled = 0;
        setup_clear_cpu_cap(X86_FEATURE_KAISER);
}

It would be better to have some logs, saying that it is disabled and
why (Xen PV).

-- Package-specific info:
** Version:
Linux version 4.9.0-5-amd64 (debian-kernel@lists.debian.org) (gcc version 6.3.0 20170516 (Debian 6.3.0-18) ) #1 SMP Debian 4.9.65-3+deb9u2 (2018-01-04)

** Command line:
root=UUID=2ae1ff60-aeb0-4d39-8a98-ff98d5f63f7e ro console=ttyS0 console=hvc0 nomce loglevel=5 net.ifnames=0

** Not tainted

** Kernel log:
Unable to read kernel log; any relevant messages should be attached

** Model information

** Loaded modules:
ipt_REJECT
nf_reject_ipv4
xt_multiport
iptable_filter
evdev
pcspkr
intel_rapl
sb_edac
edac_core
x86_pkg_temp_thermal
coretemp
crct10dif_pclmul
crc32_pclmul
ghash_clmulni_intel
intel_rapl_perf
ip_tables
x_tables
autofs4
ext4
crc16
jbd2
crc32c_generic
fscrypto
ecb
mbcache
crc32c_intel
aesni_intel
xen_netfront
xen_blkfront
aes_x86_64
glue_helper
lrw
gf128mul
ablk_helper
cryptd

** PCI devices:
not available

** USB devices:
not available


-- System Information:
Debian Release: 9.3
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-debug'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-5-amd64 (SMP w/1 CPU core)
Locale: LANG=POSIX, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=POSIX (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages linux-image-4.9.0-5-amd64 depends on:
ii  initramfs-tools [linux-initramfs-tool]  0.130
ii  kmod                                    23-2
ii  linux-base                              4.5

Versions of packages linux-image-4.9.0-5-amd64 recommends:
ii  firmware-linux-free  3.4
ii  irqbalance           1.1.0-2.3

Versions of packages linux-image-4.9.0-5-amd64 suggests:
pn  debian-kernel-handbook  <none>
ii  grub-efi-amd64          2.02~beta3-5
pn  linux-doc-4.9           <none>

Versions of packages linux-image-4.9.0-5-amd64 is related to:
pn  firmware-amd-graphics     <none>
pn  firmware-atheros          <none>
pn  firmware-bnx2             <none>
pn  firmware-bnx2x            <none>
pn  firmware-brcm80211        <none>
pn  firmware-cavium           <none>
pn  firmware-intel-sound      <none>
pn  firmware-intelwimax       <none>
pn  firmware-ipw2x00          <none>
pn  firmware-ivtv             <none>
pn  firmware-iwlwifi          <none>
pn  firmware-libertas         <none>
pn  firmware-linux-nonfree    <none>
pn  firmware-misc-nonfree     <none>
pn  firmware-myricom          <none>
pn  firmware-netxen           <none>
pn  firmware-qlogic           <none>
pn  firmware-realtek          <none>
pn  firmware-samsung          <none>
pn  firmware-siano            <none>
pn  firmware-ti-connectivity  <none>
pn  xen-hypervisor            <none>

-- no debconf information

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

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Bug#887061: linux-image-4.9.0-5-amd64: KPTI (kaiser.c): please, no silent disable for Xen guests Vincent Lefevre <vincent@vinc17.net> - 2018-01-13 10:40 +0100
  Bug#887061: marked as done (linux-image-4.9.0-5-amd64: KPTI  (kaiser.c): please, no silent disable for Xen guests) "Debian Bug Tracking System" <owner@bugs.debian.org> - 2018-01-14 21:00 +0100

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