Re: [Qemu-devel] [RFC PATCH] QEMU may write to system_memory before guest starts

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Юрий Котов (yury-kotov@yandex-team.ru) wrote:
> Ping

Is this fixed by Catherine Ho's patch series?

Dave

> 21.03.2019, 19:27, "Yury Kotov" <yury-kotov@yandex-team.ru>:
> > Hi,
> >
> > 19.03.2019, 14:52, "Dr. David Alan Gilbert" <dgilbert@redhat.com>:
> >>  * Peter Maydell (peter.maydell@linaro.org) wrote:
> >>>   On Tue, 19 Mar 2019 at 11:03, Dr. David Alan Gilbert
> >>>   <dgilbert@redhat.com> wrote:
> >>>   >
> >>>   > * Peter Maydell (peter.maydell@linaro.org) wrote:
> >>>   > > I didn't think migration distinguished between "main memory"
> >>>   > > and any other kind of RAMBlock-backed memory ?
> >>>   >
> >>>   > In Yury's case there's a distinction between RAMBlock's that are mapped
> >>>   > with RAM_SHARED (which normally ends up as MAP_SHARED) and all others.
> >>>   > You can set that for main memory by using -numa to specify a memdev
> >>>   > that's backed by a file and has the share=on property.
> >>>   >
> >>>   > On x86 the ROMs end up as separate RAMBlock's that aren't affected
> >>>   > by that -numa/share=on - so they don't fight Yury's trick.
> >>>
> >>>   You can use the generic loader on x86 to load an ELF file
> >>>   into RAM if you want, which would I think also trigger this.
> >>
> >>  OK, although that doesn't worry me too much - since in the majority
> >>  of cases Yury's trick still works well.
> >>
> >>  I wonder if there's a way to make Yury's code to detect these cases
> >>  and not allow the feature; the best thing for the moment would seem to
> >>  be to skip the aarch test that uses elf loading.
> >
> > Currently, I've no idea how to detect such cases, but there is an ability to
> > detect memory corruption. I want to update the RFC patch to let user to map some
> > memory regions as readonly until incoming migration start.
> >
> > E.g.
> > 1) If x-ignore-shared is enabled in command line or memory region is marked
> >    (something like ',readonly=on'),
> > 2) Memory region is shared (,share=on),
> > 3) And qemu is started with '-incoming' option
> >
> > Then map such regions as readonly until incoming migration finished.
> > Thus, the patch will be able to detect memory corruption and will not affect
> > normal cases.
> >
> > How do you think, is it needed?
> >
> > I already have a cleaner version of the RFC patch, but I'm not sure about 1).
> > Which way is better: enable capability in command line, add a new option for
> > memory-backend or something else.
> >
> >>  Dave
> >>
> >>>   thanks
> >>>   -- PMM
> >>  --
> >>  Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> > Regards,
> > Yury
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK