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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=195.135.220.29; envelope-from=lma@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org, dgilbert@redhat.com, quintela@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 在 2021-10-15 16:28，Peter Xu 写道： > On Fri, Oct 15, 2021 at 04:16:15PM +0800, lma wrote: >> 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. > > No worry. :) > >> >> > 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. > > I can't say it's wrong, it's just that it may need some more thoughts > on how to > make it applicable. > > We'll need to make sure no kernel module will access guest pages, > however I > think it'll be so hard to guarantee. For example, there can be some > read() > syscall from qemu initiated with guest pages passed in as the buffer > (so the > kernel will fill up the buffer when syscall returns), then if that page > is > missing on dst then that'll also trigger a kernel page fault and it'll > crash > qemu too even if no kvm/vhost-kernel is used. We'll need to dig out > everything > like that. Yeah, It's hard to avoid pf in kernel completely. > The other thing is about my original question on whether it'll be > useful in any > way, and I just worry it won't help anyone, because afaiu any real user > of > migration (I believe it's majorly public/private cloud) will definitely > at > least be kvm based as tcg could be too slow. Then they'll simply > enable the > unprivileged uffd on the hosts, since even if it's unsafe it'll be at > least as > unsafe as before unprivileged_userfaultfd is introduced. It seems that this capability is useless for qemu/kvm so far :-) Thanks for your information! Lin