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HDD write speed 3x slowed down when boot with Xen kernel

Hi. Hope someone can help with this issue.
Fresh installed server with Debian Buster on top of nvme swRaid1 (mdadm)
Testing hdd write seed with dd (with convert=fdatasync option) gives me the result of 330MB/s. Good.
Installed xen-system and xen-tools (with --no-recommends option in apt) from official repository. Rebooted the system.
hdd write seed with dd (with convert=fdatasync option) gives me the result of 108MB/s. Not good at all.

Maybe the following is not related to the issue, but on dmesg there is a line when I boot the system with Xen kernel:
[   14.214044] Performance Events: unsupported p6 CPU model 158 no PMU driver, software events only.

Instead, when I boot the system without Xen kernel I have these lines in dmesg:
[    0.517217] Performance Events: PEBS fmt3+, Skylake events, 32-deep LBR, full-width counters, Intel PMU driver.
[    0.517356] ... version:                4
[    0.517444] ... bit width:              48
[    0.517444] ... generic registers:      4
[    0.517444] ... value mask:             0000ffffffffffff
[    0.517444] ... max period:             00007fffffffffff
[    0.517444] ... fixed-purpose events:   3
[    0.517444] ... event mask:             000000070000000f

Any suggestion?
Many thanks



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