linux-kernel - Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy

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Message-ID: <20210518190049.7e6e661f@ibm-vm>
Date:   Tue, 18 May 2021 19:00:49 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        frankja@...ux.ibm.com, thuth@...hat.com, pasic@...ux.ibm.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy

On Tue, 18 May 2021 18:55:56 +0200
Christian Borntraeger <borntraeger@...ibm.com> wrote:

> On 18.05.21 18:31, Claudio Imbrenda wrote:
> > On Tue, 18 May 2021 18:22:42 +0200
> > David Hildenbrand <david@...hat.com> wrote:
> >   
> >> On 18.05.21 18:19, Claudio Imbrenda wrote:  
> >>> On Tue, 18 May 2021 18:04:11 +0200
> >>> Cornelia Huck <cohuck@...hat.com> wrote:
> >>>      
> >>>> On Tue, 18 May 2021 17:36:24 +0200
> >>>> Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:
> >>>>     
> >>>>> On Tue, 18 May 2021 17:05:37 +0200
> >>>>> Cornelia Huck <cohuck@...hat.com> wrote:
> >>>>>         
> >>>>>> On Mon, 17 May 2021 22:07:47 +0200
> >>>>>> Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:  
> >>>>     
> >>>>>>> This means that the same address space can have memory
> >>>>>>> belonging to more than one protected guest, although only one
> >>>>>>> will be running, the others will in fact not even have any
> >>>>>>> CPUs.  
> >>>>>>
> >>>>>> Are those set-aside-but-not-yet-cleaned-up pages still possibly
> >>>>>> accessible in any way? I would assume that they only belong to
> >>>>>> the  
> >>>>>
> >>>>> in case of reboot: yes, they are still in the address space of
> >>>>> the guest, and can be swapped if needed
> >>>>>         
> >>>>>> 'zombie' guests, and any new or rebooted guest is a new entity
> >>>>>> that needs to get new pages?  
> >>>>>
> >>>>> the rebooted guest (normal or secure) will re-use the same pages
> >>>>> of the old guest (before or after cleanup, which is the reason
> >>>>> of patches 3 and 4)  
> >>>>
> >>>> Took a look at those patches, makes sense.
> >>>>     
> >>>>>
> >>>>> the KVM guest is not affected in case of reboot, so the
> >>>>> userspace address space is not touched.  
> >>>>
> >>>> 'guest' is a bit ambiguous here -- do you mean the vm here, and
> >>>> the actual guest above?
> >>>>     
> >>>
> >>> yes this is tricky, because there is the guest OS, which
> >>> terminates or reboots, then there is the "secure configuration"
> >>> entity, handled by the Ultravisor, and then the KVM VM
> >>>
> >>> when a secure guest reboots, the "secure configuration" is
> >>> dismantled (in this case, in a deferred way), and the KVM VM (and
> >>> its memory) is not directly affected
> >>>
> >>> what happened before was that the secure configuration was
> >>> dismantled synchronously, and then re-created.
> >>>
> >>> now instead, a new secure configuration is created using the same
> >>> KVM VM (and thus the same mm), before the old secure configuration
> >>> has been completely dismantled. hence the same KVM VM can have
> >>> multiple secure configurations associated, sharing the same
> >>> address space.
> >>>
> >>> of course, only the newest one is actually running, the other ones
> >>> are "zombies", without CPUs.
> >>>      
> >>
> >> Can a guest trigger a DoS?  
> > 
> > I don't see how
> > 
> > a guest can fill its memory and then reboot, and then fill its
> > memory again and then reboot... but that will take time, filling
> > the memory will itself clean up leftover pages from previous boots.
> >  
> 
> In essence this guest will then synchronously wait for the page to be
> exported and reimported, correct?

correct

> > "normal" reboot loops will be fast, because there won't be much
> > memory to process
> > 
> > I have actually tested mixed reboot/shutdown loops, and the system
> > behaved as you would expect when under load.  
> 
> I guess the memory will continue to be accounted to the memcg?
> Correct?

for the reboot case, yes, since the mm is not directly affected.
for the shutdown case, I'm not sure.