Re: [RFCv1 7/7] KVM: unmap guest memory using poisoned pages

[David Hildenbrand]

On 09.04.21 16:12, Kirill A. Shutemov wrote:
> On Fri, Apr 09, 2021 at 03:50:42PM +0200, David Hildenbrand wrote:
> > > > 3. Allow selected users to still grab the pages (esp. KVM to fault them into
> > > > the page tables).
> > >
> > > As long as fault leads to non-present PTEs we are fine. Usespace still may
> > > want to mlock() some of guest memory. There's no reason to prevent this.
> >
> > I'm curious, even get_user_pages() will lead to a present PTE as is, no? So
> > that will need modifications I assume. (although I think it fundamentally
> > differs to the way get_user_pages() works - trigger a fault first, then
> > lookup the PTE in the page tables).
>
> For now, the patch has two step poisoning: first fault in, on the add to
> shadow PTE -- poison. By the time VM has chance to use the page it's
> poisoned and unmapped from the host userspace.

IIRC, this then assumes that while a page is protected, it will remain 
mapped into the NPT; because, there is no way to remap into NPT later 
because the pages have already been poisoned.

--
Thanks,

David / dhildenb