Re: [RFC PATCH 08/18] KVM: x86: Add KVM Userfault support

James Houghton
2024-07-18T17:09:11Z

On Wed, Jul 17, 2024 at 8:34 AM Wang, Wei W <wei.w.wang@intel.com> wrote:
>
> On Thursday, July 11, 2024 7:42 AM, James Houghton wrote:
> > The first prong for enabling KVM Userfault support for x86 is to be able to
> > inform userspace of userfaults. We know when userfaults occurs when
> > fault->pfn comes back as KVM_PFN_ERR_FAULT, so in
> > kvm_mmu_prepare_memory_fault_exit(), simply check if fault->pfn is indeed
> > KVM_PFN_ERR_FAULT. This means always setting fault->pfn to a known value (I
> > have chosen KVM_PFN_ERR_FAULT) before calling
> > kvm_mmu_prepare_memory_fault_exit().
> >
> > The next prong is to unmap pages that are newly userfault-enabled. Do this in
> > kvm_arch_pre_set_memory_attributes().
>
> Why is there a need to unmap it?
> I think a userfault is triggered on a page during postcopy when its data has not yet
> been fetched from the source, that is, the page is never accessed by guest on the
> destination and the page table leaf entry is empty.
>

You're right that it's not strictly necessary for implementing
post-copy. This just comes down to the UAPI we want: does
ATTRIBUTE_USERFAULT mean "KVM will be unable to access this memory;
any attempt to access it will generate a userfault" or does it mean
"accesses to never-accessed, non-prefaulted memory will generate a
userfault."

I think the former (i.e., the one implemented in this RFC) is slightly
clearer and slightly more useful.

Userfaultfd does the latter:
1. MAP_PRIVATE|MAP_ANONYMOUS + UFFDIO_REGISTER_MODE_MISSING: if
nothing is mapped (i.e., major page fault)
2. non-anonymous VMA + UFFDIO_REGISTER_MODE_MISSING: if the page cache
does not contain a page
3. MAP_SHARED + UFFDIO_REGISTER_MODE_MINOR: if the page cache
*contains* a page, but we got a fault anyway

But in all of these cases, we have a way to start getting userfaults
for already-accessed memory: for (1) and (3), MADV_DONTNEED, and for
(2), fallocate(FALLOC_FL_PUNCH_HOLE).

Even if we didn't have MADV_DONTNEED (as used to be the case with
HugeTLB), we can use PROT_NONE to prevent anyone from mapping anything
in between an mmap() and a UFFDIO_REGISTER. This has been useful for
me.

With KVM, we have neither of these tools (unless we include them here), AFAIA.