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

[Peter Xu]

On Fri, Oct 15, 2021 at 01:38:06PM +0800, lma wrote:
> 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.

I didn't follow why you thought my question was about "user-mode emulation"..

To ask in another way: after this new cap set, qemu will get a SIGBUS and VM
will crash during postcopy migrating as long as either KVM or vhost-kernel
faulted on any of the missing pages, am I right?

Thanks,

-- 
Peter Xu