Re: [RFC 0/9] Add an interVM memory sharing device

[Stefan Hajnoczi]

On Mon, Feb 10, 2020 at 02:01:48PM +0100, Igor Kotrasiński wrote:
> On 2/7/20 5:33 PM, Stefan Hajnoczi wrote:
> > On Fri, Feb 07, 2020 at 11:04:03AM +0100, Igor Mammedov wrote:
> >> On Fri, 7 Feb 2020 10:00:50 +0100
> >> Igor Kotrasiński <address@hidden> wrote:
> >>
> >>> On 2/5/20 3:49 PM, Jan Kiszka wrote:
> >>>> On 05.02.20 15:39, Stefan Hajnoczi wrote:
> >>>>> On Tue, Feb 04, 2020 at 12:30:42PM +0100,
> >>>>> address@hidden wrote:
> >>>>>> From: Igor Kotrasinski <address@hidden>
> >>>>>>
> >>>>>> This patchset adds a "memory exposing" device that allows two QEMU
> >>>>>> instances to share arbitrary memory regions. Unlike ivshmem, it does 
> >>>>>> not
> >>>>>> create a new region of memory that's shared between VMs, but instead
> >>>>>> allows one VM to access any memory region of the other VM we choose to
> >>>>>> share.
> >>>>>>
> >>>>>> The motivation for this device is a sort of ARM Trustzone "emulation",
> >>>>>> where a rich system running on one machine (e.g. x86_64 linux) is able
> >>>>>> to perform SMCs to a trusted system running on another (e.g. OpTEE on
> >>>>>> ARM). With a device that allows sharing arbitrary memory between VMs,
> >>>>>> this can be achieved with minimal changes to the trusted system and its
> >>>>>> linux driver while allowing the rich system to run on a speedier x86
> >>>>>> emulator. I prepared additional patches for linux, OpTEE OS and OpTEE
> >>>>>> build system as a PoC that such emulation works and passes OpTEE tests;
> >>>>>> I'm not sure what would be the best way to share them.
> >>>>>>
> >>>>>> This patchset is my first foray into QEMU source code and I'm certain
> >>>>>> it's not yet ready to be merged in. I'm not sure whether memory sharing
> >>>>>> code has any race conditions or breaks rules of working with memory
> >>>>>> regions, or if having VMs communicate synchronously via chardevs is the
> >>>>>> right way to do it. I do believe the basic idea for sharing memory
> >>>>>> regions is sound and that it could be useful for inter-VM 
> >>>>>> communication.
> >>>>>
> >>>>> Hi,
> >>>>> Without having looked into the patches yet, I'm already wondering if you
> >>>>> can use the existing -object
> >>>>> memory-backend-file,size=512M,mem-path=/my/shared/mem feature for your
> >>>>> use case?
> >>>>>
> >>>>> That's the existing mechanism for fully sharing guest RAM and if you
> >>>>> want to share all of memory then maybe a device is not necessary - just
> >>>>> share the memory.
> >>>
> >>> That option adds memory in addition to the memory allocated with the
> >>> '-m' flag, doesn't it? I looked into that option, and it seemed to me
> >>> you can't back all memory this way.
> >> with current QEMU you play with memory sharing using numa workaround
> >>
> >> -m 512 \
> >> -object memory-backend-file,id=mem,size=512M,mem-path=/my/shared/mem 
> >> feature,share=on \
> >> -numa node,memdev=mem
> >>
> >> also on the list there is series that allows to share main ram
> >> without numa workaround, see
> >>    "[PATCH v4 00/80] refactor main RAM allocation to use hostmem backend"
> >>
> >> with it applied you can share main RAM with following CLI:
> >>
> >> -object memory-backend-file,id=mem,size=512M,mem-path=/my/shared/mem 
> >> feature,share=on \
> >> -m 512 \
> >> -M virt,memory-backend=mem
> > 
> > Nice!  That takes care of memory.
> 
> After a bit of hacking to map the shared RAM instead of communicating 
> via socket I can confirm - I can run OpTEE this way, and it passes 
> tests. My solution is *technically* more accurate since it is aware of 
> memory subregions and completely independent from memory backend setup, 
> but with my use case satisfied already, I don't think it's of use to anyone.

Great!

Stefan

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