> Date: Wed, 18 Mar 2015 21:23:48 -0300 > From: Marcelo Tosatti <mtosatti@xxxxxxxxxx> > > On Mon, Mar 16, 2015 at 09:51:40AM +0100, Christian Borntraeger wrote: > > From: Thomas Huth <thuth@xxxxxxxxxxxxxxxxxx> > > > > 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. Thomas -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
