Re: [PATCH/RFC 4/9] KVM: s390: Add MEMOP ioctls for reading/writing guest memory

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Mar 19, 2015 at 02:15:18PM +0100, Thomas Huth wrote:
> > Date: Wed, 18 Mar 2015 21:23:48 -0300
> > From: Marcelo Tosatti <mtosatti@redhat.com>
> >
> > On Mon, Mar 16, 2015 at 09:51:40AM +0100, Christian Borntraeger wrote:
> > > From: Thomas Huth <thuth@linux.vnet.ibm.com>
> > >
> > > On s390, we've got to make sure to hold the IPTE lock while accessing
> > > logical memory. So let's add an ioctl for reading and writing logical
> > > memory to provide this feature for userspace, too.
> ...
> >
> > What i was wondering is why you can't translate the address
> > in the kernel and let userspace perform the actual read/write?
> 
> The idea here is to protect the read/write access with the ipte-lock, too.
> That way, the whole address translation _and_ the read/write access are
> protected together against invalidate-page-table operations from other
> CPUs,
> so the whole memory access looks atomic for other VCPUs. And since we do
> not
> want to expose the ipte lock directly to user space, both has to be done
> in the kernel.
> We already had a long internal discussion about this in our team, and
> indeed, if the ipte-lock would be the only "problem" that we face on s390,
> we might also come up with a solution where the memory read/write access
> is done in userspace instead. However, for full architecture compliance,
> we later have got to support the so-called "storage keys" during memory
> accesses, too, and this can hardly be done accurately and safely from
> userspace. So I'm afraid, it's somewhat ugly that we've got to provide an
> ioctl here to read/write the guest memory, but it's the only feasible
> solution that I could think of.

I see, thanks.