Re: [PATCH V1 18/32] osdep: import MADV_DOEXEC

[Steven Sistare]

On 7/30/2020 11:14 AM, Steve Sistare wrote:
> Anonymous memory segments used by the guest are preserved across a re-exec
> of qemu, mapped at the same VA, via a proposed madvise(MADV_DOEXEC) option
> in the Linux kernel. For the madvise patches, see:
> 
> https://lore.kernel.org/lkml/1595869887-23307-1-git-send-email-anthony.yznaga@oracle.com/
> 
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>  include/qemu/osdep.h | 7 +++++++
>  1 file changed, 7 insertions(+)

Hi Alex,
  The MADV_DOEXEC functionality, which is a pre-requisite for the entire qemu 
live update series, is getting a chilly reception on lkml.  We could instead 
create guest memory using memfd_create and preserve the fd across exec.  
However, 
the subsequent mmap(fd) will return a different VA than was used previously, 
which  is a problem for memory that was registered with vfio, as the original 
VA 
is remembered in the kernel struct vfio_dma and used in various kernel 
functions, 
such as vfio_iommu_replay.

To fix, we could provide a VFIO_IOMMU_REMAP_DMA ioctl taking iova, size, and
new_vaddr.  The implementation finds an exact match for (iova, size) and 
replaces 
vaddr with new_vaddr.  Flags cannot be changed.

memfd_create plus VFIO_IOMMU_REMAP_DMA would replace MADV_DOEXEC.
vfio on any form of shared memory (shm, dax, etc) could also be preserved across
exec with shmat/mmap plus VFIO_IOMMU_REMAP_DMA.

What do you think?

- Steve