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. This field has a 16-bit size to accommodate into the existing
padding virtio_pci_cap, simplifying the introduction of this additional
data into the structure. The device will provide the page size in format
PAGE_SIZE >> 12.
The series also extends the PCI and MMIO transports to obtain the
corresponding value from the device. For the PCI one, it should be safe
since we're using an existing 16-bit padding in the virtio_pci_cap
struct. For MMIO, we need to access a new register, so there's a risk
the VMM may overreact and crash the VM. I've checked libkrun,
firecracker, cloud-hypervisor and crosvm, and all of them should deal
with the unexpected MMIO read gracefully. QEMU doesn't support SHM for
the MMIO transport, so that isn't a concern either.
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 <slp@xxxxxxxxxx>
---
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 to hold page_size
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 | 13 +++++++++++++
drivers/virtio/virtio_pci_modern.c | 31 ++++++++++++++++++++++++++++---
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 | 2 +-
9 files changed, 60 insertions(+), 5 deletions(-)
---
base-commit: 4dc1d1bec89864d8076e5ab314f86f46442bfb02
change-id: 20250213-virtio-shm-page-size-6e9a08c7ded1
Best regards,
--
Sergio Lopez <slp@xxxxxxxxxx>
