Re: [RFC PATCH 0/5] virtio: obtain SHM page size from device

[Michael S. Tsirkin]

On Thu, Feb 13, 2025 at 04:49:14PM +0100, Sergio Lopez wrote:
> 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>


don't you want to negotiate the page size with the
driver then?

> ---
> 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>