On Thu, 2025-01-23 at 11:39 +0000, David Hildenbrand wrote:
On 23.01.25 10:48, Fuad Tabba wrote:
On Wed, 22 Jan 2025 at 22:10, David Hildenbrand <david@xxxxxxxxxx> wrote:
On 22.01.25 16:27, Fuad Tabba wrote:
Make kvm_(read|/write)_guest_page() capable of accessing guest
memory for slots that don't have a userspace address, but only if
the memory is mappable, which also indicates that it is
accessible by the host.
Interesting. So far my assumption was that, for shared memory, user
space would simply mmap() guest_memdd and pass it as userspace address
to the same memslot that has this guest_memfd for private memory.
Wouldn't that be easier in the first shot? (IOW, not require this patch
with the cost of faulting the shared page into the page table on access)
In light of:
https://lkml.kernel.org/r/20250117190938.93793-4-imbrenda@xxxxxxxxxxxxx
there can, in theory, be memslots that start at address 0 and have a
"valid" mapping. This case is done from the kernel (and on special s390x
hardware), though, so it does not apply here at all so far.
In practice, getting address 0 as a valid address is unlikely, because
the default:
$ sysctl vm.mmap_min_addr
vm.mmap_min_addr = 65536
usually prohibits it for good reason.
This has to do more with the ABI I had for pkvm and shared memory
implementations, in which you don't need to specify the userspace
address for memory in a guestmem memslot. The issue is there is no
obvious address to map it to. This would be the case in kvm:arm64 for
tracking paravirtualized time, which the userspace doesn't necessarily
need to interact with, but kvm does.
So I understand correctly: userspace wouldn't have to mmap it because it
is not interested in accessing it, but there is nothing speaking against
mmaping it, at least in the first shot.
I assume it would not be a private memslot (so far, my understanding is
that internal memslots never have a guest_memfd attached).
kvm_gmem_create() is only called via KVM_CREATE_GUEST_MEMFD, to be set
on user-created memslots.
That said, we could always have a userspace address dedicated to
mapping shared locations, and use that address when the necessity
arises. Or we could always require that memslots have a userspace
address, even if not used. I don't really have a strong preference.
So, the simpler version where user space would simply mmap guest_memfd
to provide the address via userspace_addr would at least work for the
use case of paravirtualized time?
fwiw, I'm currently prototyping something like this for x86 (although
not by putting the gmem address into userspace_addr, but by adding a new
field to memslots, so that memory attributes continue working), based on
what we talked about at the last guest_memfd sync meeting (the whole
"how to get MMIO emulation working for non-CoCo VMs in guest_memfd"
story). So I guess if we're going down this route for x86, maybe it
makes sense to do the same on ARM, for consistency?
It would get rid of the immediate need for this patch and patch #4 to
get it flying.
One interesting question is: when would you want shared memory in
guest_memfd and *not* provide it as part of the same memslot.
In my testing of non-CoCo gmem VMs on ARM, I've been able to get quite
far without giving KVM a way to internally access shared parts of gmem -
it's why I was probing Fuad for this simplified series, because
KVM_SW_PROTECTED_VM + mmap (for loading guest kernel) is enough to get a
working non-CoCo VM on ARM (although I admittedly never looked at clocks
inside the guest - maybe that's one thing that breaks if KVM can't
access gmem. How to guest and host agree on the guest memory range
used to exchange paravirtual timekeeping information? Could that exchange
be intercepted in userspace, and set to shared via memory attributes (e.g.
placed outside gmem)? That's the route I'm going down the paravirtual
time on x86).
