Re: [RFCv2 00/16] KVM protected memory extension

[Kirill A. Shutemov]

On Wed, Oct 21, 2020 at 11:20:56AM -0700, Andy Lutomirski wrote:
> > On Oct 19, 2020, at 11:19 PM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> 
> > For removing the userspace mapping, use a trick similar to what NUMA
> > balancing does: convert memory that belongs to KVM memory slots to
> > PROT_NONE: all existing entries converted to PROT_NONE with mprotect() and
> > the newly faulted in pages get PROT_NONE from the updated vm_page_prot.
> > The new VMA flag -- VM_KVM_PROTECTED -- indicates that the pages in the
> > VMA must be treated in a special way in the GUP and fault paths. The flag
> > allows GUP to return the page even though it is mapped with PROT_NONE, but
> > only if the new GUP flag -- FOLL_KVM -- is specified. Any userspace access
> > to the memory would result in SIGBUS. Any GUP access without FOLL_KVM
> > would result in -EFAULT.
> >
> 
> I definitely like the direction this patchset is going in, and I think
> that allowing KVM guests to have memory that is inaccessible to QEMU
> is a great idea.
> 
> I do wonder, though: do we really want to do this with these PROT_NONE
> tricks, or should we actually come up with a way to have KVM guest map
> memory that isn't mapped into QEMU's mm_struct at all?  As an example
> of the latter, I mean something a bit like this:
> 
> https://lkml.kernel.org/r/CALCETrUSUp_7svg8EHNTk3nQ0x9sdzMCU=h8G-Sy6=SODq5GHg@xxxxxxxxxxxxxx
> 
> I don't mean to say that this is a requirement of any kind of
> protected memory like this, but I do think we should understand the
> tradeoffs, in terms of what a full implementation looks like, the
> effort and time frames involved, and the maintenance burden of
> supporting whatever gets merged going forward.

I considered the PROT_NONE trick neat. Complete removing of the mapping
from QEMU would require more changes into KVM and I'm not really familiar
with it.

About tradeoffs: the trick interferes with AutoNUMA. I didn't put much
thought into how we can get it work together. Need to look into it.

Do you see other tradeoffs?

-- 
 Kirill A. Shutemov