Message-ID: <diqza51cjyo1.fsf@google.com>
Date: Mon, 27 Oct 2025 05:51:10 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
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Subject: Re: [RFC PATCH v1 16/37] KVM: selftests: Add support for mmap() on
 guest_memfd in core library

Sean Christopherson <seanjc@...gle.com> writes:

> On Fri, Oct 24, 2025, Ackerley Tng wrote:
>> Ackerley Tng <ackerleytng@...gle.com> writes:
>> 
>> > From: Sean Christopherson <seanjc@...gle.com>
>> >
>> > Accept gmem_flags in vm_mem_add() to be able to create a guest_memfd within
>> > vm_mem_add().
>> >
>> > When vm_mem_add() is used to set up a guest_memfd for a memslot, set up the
>> > provided (or created) gmem_fd as the fd for the user memory region. This
>> > makes it available to be mmap()-ed from just like fds from other memory
>> > sources. mmap() from guest_memfd using the provided gmem_flags and
>> > gmem_offset.
>> >
>> > Add a kvm_slot_to_fd() helper to provide convenient access to the file
>> > descriptor of a memslot.
>> >
>> > Update existing callers of vm_mem_add() to pass 0 for gmem_flags to
>> > preserve existing behavior.
>> >
>> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>> > [For guest_memfds, mmap() using gmem_offset instead of 0 all the time.]
>> > Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
>> > ---
>> >  tools/testing/selftests/kvm/include/kvm_util.h |  7 ++++++-
>> >  tools/testing/selftests/kvm/lib/kvm_util.c     | 18 ++++++++++--------
>> >  .../kvm/x86/private_mem_conversions_test.c     |  2 +-
>> >  3 files changed, 17 insertions(+), 10 deletions(-)
>> >
>> > 
>> > [...snip...]
>> > 
>> > @@ -1050,13 +1049,16 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>> >  	}
>> >  
>> >  	region->fd = -1;
>> > -	if (backing_src_is_shared(src_type))
>> > +	if (flags & KVM_MEM_GUEST_MEMFD && gmem_flags & GUEST_MEMFD_FLAG_MMAP)
>> > +		region->fd = kvm_dup(gmem_fd);
>> > +	else if (backing_src_is_shared(src_type))
>> >  		region->fd = kvm_memfd_alloc(region->mmap_size,
>> >  					     src_type == VM_MEM_SRC_SHARED_HUGETLB);
>> >  
>> 
>> Doing this makes it hard to test the legacy dual-backing case.
>> 
>> It actually broke x86/private_mem_conversions_test for the legacy
>> dual-backing case because there's no way to mmap or provide a
>> userspace_address from the memory provider that is not guest_memfd, as
>> determined by src_type.
>
> Yes there is.  This patch is a giant nop.  The only thing that the core library
> doesn't support is mmap() on guest_memfd *and* the other src_type, and IMO that
> is big "don't care", because KVM doesn't even support that combination:
>
> 	if (kvm_gmem_supports_mmap(inode))
> 		slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
>

Makes sense.

> I mean, we _could_ test that KVM ignores the hva for mapping, but that's a
> different and unique test entirely.
>
> I did break x86/private_mem_conversions_test (I could have sworn I tested, *sigh*),
> but the bug is in:
>
>   KVM: selftests: Provide function to look up guest_memfd details from gpa
>
> not here.  And it's a trivial /facepalm-style fix:
>
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index ee5b63f7cb50..23a8676fee6d 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -1680,7 +1680,7 @@ int kvm_gpa_to_guest_memfd(struct kvm_vm *vm, vm_paddr_t gpa, off_t *fd_offset,
>         gpa_offset = gpa - region->region.guest_phys_addr;
>         *fd_offset = region->region.guest_memfd_offset + gpa_offset;
>         *nr_bytes = region->region.memory_size - gpa_offset;
> -       return region->fd;
> +       return region->region.guest_memfd;
>  }
>  
>  /* Create an interrupt controller chip for the specified VM. */

This works. Thanks!

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