Re: [PATCH 0/3] Postcopy migration: Add userfaultfd- user-mode-only capability

[lma <lma@suse.de>]

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_userfaultfd
>> sysctl knob is 0.
>> Please refer to https://lwn.net/Articles/819834/ and the kernel 
>> commits:
>> 37cd0575 userfaultfd: add UFFD_USER_MODE_ONLY
>> d0d4730a userfaultfd: add user-mode only option to 
>> unprivileged_userfaultfd sysctl knob
>> 
>> This 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 real
> user 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