On Thu, Jun 19, 2014 at 12:20:32PM +0300, Gleb Natapov wrote: > CCing Marcelo, > > On Wed, Jun 18, 2014 at 02:50:44PM +0800, Tang Chen wrote: > > Hi Gleb, > > > > Thanks for the quick reply. Please see below. > > > > On 06/18/2014 02:12 PM, Gleb Natapov wrote: > > >On Wed, Jun 18, 2014 at 01:50:00PM +0800, Tang Chen wrote: > > >>[Questions] > > >>And by the way, would you guys please answer the following questions for me ? > > >> > > >>1. What's the ept identity pagetable for ? Only one page is enough ? > > >> > > >>2. Is the ept identity pagetable only used in realmode ? > > >> Can we free it once the guest is up (vcpu in protect mode)? > > >> > > >>3. Now, ept identity pagetable is allocated in qemu userspace. > > >> Can we allocate it in kernel space ? > > >What would be the benefit? > > > > I think the benefit is we can hot-remove the host memory a kvm guest > > is using. > > > > For now, only memory in ZONE_MOVABLE can be migrated/hot-removed. And the > > kernel > > will never use ZONE_MOVABLE memory. So if we can allocate these two pages in > > kernel space, we can pin them without any trouble. When doing memory > > hot-remove, > > the kernel will not try to migrate these two pages. > But we can do that by other means, no? The patch you've sent for instance. > > > > > > > > >> > > >>4. If I want to migrate these two pages, what do you think is the best way ? > > >> > > >I answered most of those here: http://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg103718.html > > > > I'm sorry I must missed this email. > > > > Seeing your advice, we can unpin these two pages and repin them in the next > > EPT violation. > > So about this problem, which solution would you prefer, allocate these two > > pages in kernel > > space, or migrate them before memory hot-remove ? > > > > I think the first solution is simpler. But I'm not quite sure if there is > > any other pages > > pinned in memory. If we have the same problem with other kvm pages, I think > > it is better to > > solve it in the second way. > > > > What do you think ? > Remove pinning is preferable. In fact looks like for identity pagetable > it should be trivial, just don't pin. APIC access page is a little bit > more complicated since its physical address needs to be tracked to be > updated in VMCS. Yes, and there are new users of page pinning as well soon (see PEBS threads on kvm-devel). Was thinking of notifiers scheme. Perhaps: ->begin_page_unpin(struct page *page) - Remove any possible access to page. ->end_page_unpin(struct page *page) - Reinstantiate any possible access to page. For KVM: ->begin_page_unpin() - Remove APIC-access page address from VMCS. or - Remove spte translation to pinned page. - Put vcpu in state where no VM-entries are allowed. ->end_page_unpin() - Setup APIC-access page, ... - Allow vcpu to VM-entry. Because allocating APIC access page from distant NUMA node can be a performance problem, i believe. I'd be happy to know why notifiers are overkill. -- 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
