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From: | lma |
Subject: | Re: [PATCH 0/3] Postcopy migration: Add userfaultfd- user-mode-only capability |
Date: | Fri, 15 Oct 2021 13:38:06 +0800 |
User-agent: | Roundcube Webmail |
在 2021-10-15 07:43,Peter Xu 写道:
On Thu, Oct 14, 2021 at 05:15:48PM +0800, Lin Ma wrote:Since kernel v5.11, Unprivileged user (without SYS_CAP_PTRACE capability) must pass UFFD_USER_MODE_ONLY to userfaultd in case unprivileged_userfaultfdsysctl knob is 0.Please refer to https://lwn.net/Articles/819834/ and the kernel commits:37cd0575 userfaultfd: add UFFD_USER_MODE_ONLYd0d4730a userfaultfd: add user-mode only option to unprivileged_userfaultfd sysctl knobThis patch set adds a migration capability to pass UFFD_USER_MODE_ONLY for postcopy migration.Then it's at least no KVM, no vhost, am I right? Could I ask is there a realuser behind this? Thanks,
Well, The "user-mode-only" has nothing to do with qemu's user-mode emulation.
The unprivileged_userfaultfd sysctl knob controls whether unprivileged users can use the userfaultfd system calls. set it to 1 to allow unprivileged users to use the userfaultfd system calls. set it to 0 to restrict userfaultfd to only privileged users (with SYS_CAP_PTRACE capability).
If host's unprivileged_userfaultfd sysctl knob is 0(The default value of this knob is changed to 0 since host kernel v5.11): Qemu must pass the UFFD_USER_MODE_ONLY flag when creating userfaultfd object for postcopy migration in case qemu runs as unprivileged user.
Before host kernel v5.11, If host's unprivileged_userfaultfd sysctl knob is 0, Then postcopy migration is not allowed in case qemu runs as unprivileged user.
Thanks, Lin
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