[RFC,v2,0/5] virtio: obtain SHM page size from device

Message

Sergio Lopez Feb. 14, 2025, 3:16 p.m. UTC

There's an incresing number of machines supporting multiple page sizes
and, on these machines, the host and a guest can be running with
different pages sizes.

In addition to this, there might be devices that have a required and/or
preferred page size for mapping memory.

In this series, we extend virtio_shm_region with a field to hold the
page size. The PCI and MMIO transports are updated to read the page
shift for each region and derive the page size accordingly. For the PCI
transport we're borrowing 8 bits from the 16 bit padding, so the
transition is seamless. But for MMIO we need to access a new register,
which could potentially be interpreted by the VMM as an error. For this
reason we're gating this behavior, for every transport, behind the
VIRTIO_F_SHM_PAGE_SIZE feature bit.

How the SHM page size information is used depends on each device. Some
may silently round up allocations, some may expose this information to
userspace. This series includes a patch that extends virtio-gpu to
expose the information via the VIRTGPU_GETPARAM ioctl, as an example of
the second approach.

This patch series is an RFC because it requires changes to the VIRTIO
specifications. This patch series will be used as a reference to
propose such changes.

Signed-off-by: Sergio Lopez <[e-mail]>
---
Changes in v2:
- Use "page_shift" instead of "page_size" in the transport-related
  structures to clarify its contents (thanks Daniel Verkamp).
- Update the command for "page_shift" in "virtio_pci_cap" to clarify
  that it's only relevant for VIRTIO_PCI_CAP_SHARED_MEMORY_CFG regions.
  (thanks Daniel Verkamp).
- Instead of a derived value, store page size in virtio_shm_region.
- Update the cover page to reflect that the functionality is gated
  behind the VIRTIO_F_SHM_PAGE_SIZE feature bit.
- Link to v1: https://lore.kernel.org/r/[e-mail]

---
Sergio Lopez (5):
      virtio_config: add page_size field to virtio_shm_region
      virtio: introduce VIRTIO_F_SHM_PAGE_SIZE
      virtio-pci: extend virtio_pci_cap with page_shift
      virtio-mmio: read shm region page size
      drm/virtio: add VIRTGPU_PARAM_HOST_SHM_PAGE_SIZE to params

 drivers/gpu/drm/virtio/virtgpu_ioctl.c |  5 +++++
 drivers/virtio/virtio_mmio.c           | 11 +++++++++++
 drivers/virtio/virtio_pci_modern.c     | 24 ++++++++++++++++++++----
 drivers/virtio/virtio_ring.c           |  2 ++
 include/linux/virtio_config.h          |  1 +
 include/uapi/drm/virtgpu_drm.h         |  1 +
 include/uapi/linux/virtio_config.h     |  7 ++++++-
 include/uapi/linux/virtio_mmio.h       |  3 +++
 include/uapi/linux/virtio_pci.h        |  3 ++-
 9 files changed, 51 insertions(+), 6 deletions(-)
---
base-commit: 4dc1d1bec89864d8076e5ab314f86f46442bfb02
change-id: 20250213-virtio-shm-page-size-6e9a08c7ded1

Best regards,