Re: [PATCH 10/12] mm: x86: Invoke hypercall when page encryption status is changed

[Ashish Kalra]

On Fri, Feb 14, 2020 at 10:56:53AM -0800, Andy Lutomirski wrote:
> On Thu, Feb 13, 2020 at 2:28 PM Ashish Kalra <ashish.kalra@xxxxxxx> wrote:
> >
> > On Wed, Feb 12, 2020 at 09:42:02PM -0800, Andy Lutomirski wrote:
> > >> On Wed, Feb 12, 2020 at 5:18 PM Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
> > >> >
> > >> > From: Brijesh Singh <brijesh.singh@xxxxxxx>
> > > >
> > > > Invoke a hypercall when a memory region is changed from encrypted ->
> > > > decrypted and vice versa. Hypervisor need to know the page encryption
> > > > status during the guest migration.
> > >>
> > >> What happens if the guest memory status doesn't match what the
> > >> hypervisor thinks it is?  What happens if the guest gets migrated
> > >> between the hypercall and the associated flushes?
> >
> > This is basically same as the dirty page tracking and logging being done
> > during Live Migration. As with dirty page tracking and logging we
> > maintain a page encryption bitmap in the kernel which keeps tracks of
> > guest's page encrypted/decrypted state changes and this bitmap is
> > sync'ed regularly from kernel to qemu and also during the live migration
> > process, therefore any dirty pages whose encryption status will change
> > during migration, should also have their page status updated when the
> > page encryption bitmap is sync'ed.
> >
> > Also i think that when the amount of dirty pages reach a low threshold,
> > QEMU stops the source VM and then transfers all the remaining dirty
> > pages, so at that point, there will also be a final sync of the page
> > encryption bitmap, there won't be any hypercalls after this as the
> > source VM has been stopped and the remaining VM state gets transferred.
> 
> And have you ensured that, in the inevitable race when a guest gets
> migrated part way through an encryption state change, that no data
> corruption occurs?

We ensure that we send the page encryption state bitmap to the
destination VM at migration completion and when the remaining amount of 
RAM/dirty pages are flushed. Also as the source VM is stopped before
this flush of remaining blocks occur, so any encryption state change
hypercalls would have been completed before that.

Thanks,
Ashish