> > > > > > > > > > > > On Thu 10-01-19 12:26:17, Dave Chinner wrote: > > > > On Wed, Jan 09, 2019 at 08:17:31PM +0530, Pankaj Gupta wrote: > > > > > This patch series has implementation for "virtio pmem". > > > > > "virtio pmem" is fake persistent memory(nvdimm) in guest > > > > > which allows to bypass the guest page cache. This also > > > > > implements a VIRTIO based asynchronous flush mechanism. > > > > > > > > Hmmmm. Sharing the host page cache direct into the guest VM. Sounds > > > > like a good idea, but..... > > > > > > > > This means the guest VM can now run timing attacks to observe host > > > > side page cache residency, and depending on the implementation I'm > > > > guessing that the guest will be able to control host side page > > > > cache eviction, too (e.g. via discard or hole punch operations). > > > > > > > > Which means this functionality looks to me like a new vector for > > > > information leakage into and out of the guest VM via guest > > > > controlled host page cache manipulation. > > > > > > > > https://arxiv.org/pdf/1901.01161 > > > > > > > > I might be wrong, but if I'm not we're going to have to be very > > > > careful about how guest VMs can access and manipulate host side > > > > resources like the page cache..... > > > > > > Right. Thinking about this I would be more concerned about the fact that > > > guest can effectively pin amount of host's page cache upto size of the > > > device/file passed to guest as PMEM, can't it Pankaj? Or is there some > > > QEMU > > > magic that avoids this? > > > > Yes, guest will pin these host page cache pages using 'get_user_pages' by > > elevating the page reference count. But these pages can be reclaimed by > > host > > at any time when there is memory pressure. > > Wait, how can the guest pin the host pages? I would expect this to > happen only when using vfio and device assignment. Otherwise, no the > host can't reclaim a pinned page, that's the whole point of a pin to > prevent the mm from reclaiming ownership. yes. You are right I just used the pin word but it does not actually pin pages permanently. I had gone through the discussion on existing problems with get_user_pages and DMA e.g [1] to understand Jan's POV. It does mention GUP pin pages so I also used the word 'pin'. But guest does not permanently pin these pages and these pages can be reclaimed by host. > > > KVM does not permanently pin pages. vfio does that but we are not using > > it here. > > Right, so I'm confused by your pin assertion above. Sorry! for the confusion. [1] https://lwn.net/Articles/753027/ Thanks, Pankaj
