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

[lma]

在 2021-10-15 14:12，Peter Xu 写道：
> 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"..
Sorry about the misunderstanding.

> 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?

Oops...Yes, you're right. It indeed casues qemu crash on destination due 
to
fault on missing pages.
This patch set and my thought about introducing this cap to qemu are 
wrong.

Thanks,
Lin