On Tue, Feb 19, 2019 at 09:21:20PM +0100, David Hildenbrand wrote: > On 19.02.19 21:17, Michael S. Tsirkin wrote: > > On Tue, Feb 19, 2019 at 09:02:52PM +0100, David Hildenbrand wrote: > >> On 19.02.19 20:58, Michael S. Tsirkin wrote: > >>> On Tue, Feb 19, 2019 at 10:06:35AM -0800, Alexander Duyck wrote: > >>>>> I tend to like an asynchronous reporting approach as discussed in this > >>>>> thread, we would have to see if Nitesh could get it implemented. > >>>> > >>>> I agree it would be great if it could work. However I have concerns > >>>> given that work on this patch set dates back to 2017, major issues > >>>> such as working around device assignment have yet to be addressed, > >>> > >>> BTW for device assignment to work, your idea of sending > >>> data directly to kvm won't work, will it? > >>> You need to update userspace so it can update VFIO right? > >>> Another blocker for assignment is ability to make holes > >>> an an existing mapping - supported by hardware but > >>> not by IOMMU drivers. > >> > >> I had the exact same thought and then realized that we decided to block > >> the balloon in user space until we figured out how to handle this properly. > >> > >> I wonder if MADV_FREE behaves differently compared to MADV_DONTNEED when > >> finding pinned pages, but I doubt it. Most probably we'll have to > >> disable hinting for device assignments as well. > > > > OK but let's recognize it as a bug not a feature. > > > > Yes, btw interesting read: https://lwn.net/Articles/198380/ There's also slideware from Rik van Riel circa 2011. His idea was tagging pages in guest memory, freeing a page involves - drop the EPT PTE - check page is still free - unpin page This way you can hint without exits at all if you like. > > "Pages which have been locked into memory pose an extra challenge here - > they can be part of the page cache, but they still shouldn't be taken > away by the host system. So such pages cannot be marked as "volatile." > The problem is that figuring out if a page is locked is harder than it > might seem; it can involve scanning a list of virtual memory area (VMA) > structures, which is slow. So the hinting patches add a new flag to the > address_space structure to note that somebody has locked pages from that > address space in memory." > > I assume locked here actually means pinned. Locked seems to mean mlock there. This seems to also resemble Xen's tmem a bit. > -- > > Thanks, > > David / dhildenb
