Rusty Russell wrote: > On Tue, 2007-07-24 at 08:30 +0300, Avi Kivity wrote: > >> Rusty Russell wrote: >> >>> On Mon, 2007-07-23 at 13:27 +0300, Avi Kivity wrote: >>> >>> >>>> Having an address_space (like your patch does) is remarkably simple, and >>>> requires few hooks from the current vm. However using existing vmas >>>> mapped by the user has many advantages: >>>> >>>> - compatible with s390 requirements >>>> - allows the user to use hugetlbfs pages, which have a performance >>>> advantage using ept/npt (but which are unswappable) >>>> - allows the user to map a file (which can be regarded as way to specify >>>> the swap device) >>>> - better ingration with the rest of the vm >>>> >>>> >>> You don't need to expose the vmas. You just have userspace point out >>> the start+len of each region of memory it wants the guest to be able to >>> access, and the address it wants it to appear in the guest. >>> >>> This is a slight superset of what lguest does in two ways: >>> >>> 1) my guest address == user address, but I'm looking at adding an offset >>> so I don't have to link the launcher binary specially. >>> 2) I have only one contiguous region of guest-physical memory, since I >>> can place device memory immediately above "normal" mem. >>> >>> >>> >> My intent was to allow userspace to establish assign a virtual address >> range into a memory slot. >> >> So long as you don't do swapping, all is simple, since you can do a >> get_user_pages() on initialization or when installing a shadow pte. But >> if you want to swap, you need: >> >> - a way to transfer the dirty bit from the shadow ptes to the struct page >> > > Actually, get_user_pages() does that for you. You have to make R/O any > writable pte where the guest doesn't set the dirty bit (so you can trap > it later) but last I put a printk in there, Linux doesn't do that. > >
Don't understand. You mean Linux always sets the dirty bit when it makes a page writable? Surely some mistake. It probably does do so on demand write faults, but I'm sure the dirty bit can get cleaned out by the swapper. >> - a way to let the vm rmap know that there are shadow ptes that point to >> the page in addition to Linux ptes. These shadow ptes may be in a >> different format than Linux ptes. >> - a different tlb invalidation method with ASIDs >> > > Well first I was just going to see how well hooking into the shrinker > works. That might be sufficient: just throw out shadow refs to pages > when there's pressure. > Ah, interesting. Yes, you trim the shadow page table cache which unrefs pages for you. Maybe that's a good way to get things started. > If not, it does get harder. A callback in the mm struct to say "I want > to swap your page out" is required if we don't take a reference to the > page. Dirty bit handling would be an interesting issue (maybe the > callback can say "No!" and dirty the page again?). > Since we have rmap, I don't see that as an issue. Given a page, we can easily drop all refs. Though lguest doesn't do that, right? I'm also concerned with picking the correct page, but there's no good solution here. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
