Re: [PATCH v15 3/5] vpci: add initial support for virtual PCI bus topology

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, May 17, 2024 at 01:06:13PM -0400, Stewart Hildebrand wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> 
> Assign SBDF to the PCI devices being passed through with bus 0.
> The resulting topology is where PCIe devices reside on the bus 0 of the
> root complex itself (embedded endpoints).
> This implementation is limited to 32 devices which are allowed on
> a single PCI bus.
> 
> Please note, that at the moment only function 0 of a multifunction
> device can be passed through.
> 
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
> Acked-by: Jan Beulich <jbeulich@suse.com>
> ---
> In v15:
> - add Jan's A-b
> In v13:
> - s/depends on/select/ in Kconfig
> - check pdev->sbdf.fn instead of two booleans in add_virtual_device()
> - comment #endifs in sched.h
> - clarify comment about limits in vpci.h with seg/bus limit
> In v11:
> - Fixed code formatting
> - Removed bogus write_unlock() call
> - Fixed type for new_dev_number
> In v10:
> - Removed ASSERT(pcidevs_locked())
> - Removed redundant code (local sbdf variable, clearing sbdf during
> device removal, etc)
> - Added __maybe_unused attribute to "out:" label
> - Introduced HAS_VPCI_GUEST_SUPPORT Kconfig option, as this is the
>   first patch where it is used (previously was in "vpci: add hooks for
>   PCI device assign/de-assign")
> In v9:
> - Lock in add_virtual_device() replaced with ASSERT (thanks, Stewart)
> In v8:
> - Added write lock in add_virtual_device
> Since v6:
> - re-work wrt new locking scheme
> - OT: add ASSERT(pcidevs_write_locked()); to add_virtual_device()
> Since v5:
> - s/vpci_add_virtual_device/add_virtual_device and make it static
> - call add_virtual_device from vpci_assign_device and do not use
>   REGISTER_VPCI_INIT machinery
> - add pcidevs_locked ASSERT
> - use DECLARE_BITMAP for vpci_dev_assigned_map
> Since v4:
> - moved and re-worked guest sbdf initializers
> - s/set_bit/__set_bit
> - s/clear_bit/__clear_bit
> - minor comment fix s/Virtual/Guest/
> - added VPCI_MAX_VIRT_DEV constant (PCI_SLOT(~0) + 1) which will be used
>   later for counting the number of MMIO handlers required for a guest
>   (Julien)
> Since v3:
>  - make use of VPCI_INIT
>  - moved all new code to vpci.c which belongs to it
>  - changed open-coded 31 to PCI_SLOT(~0)
>  - added comments and code to reject multifunction devices with
>    functions other than 0
>  - updated comment about vpci_dev_next and made it unsigned int
>  - implement roll back in case of error while assigning/deassigning devices
>  - s/dom%pd/%pd
> Since v2:
>  - remove casts that are (a) malformed and (b) unnecessary
>  - add new line for better readability
>  - remove CONFIG_HAS_VPCI_GUEST_SUPPORT ifdef's as the relevant vPCI
>     functions are now completely gated with this config
>  - gate common code with CONFIG_HAS_VPCI_GUEST_SUPPORT
> New in v2
> ---
>  xen/drivers/Kconfig     |  4 +++
>  xen/drivers/vpci/vpci.c | 57 +++++++++++++++++++++++++++++++++++++++++
>  xen/include/xen/sched.h | 10 +++++++-
>  xen/include/xen/vpci.h  | 12 +++++++++
>  4 files changed, 82 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/drivers/Kconfig b/xen/drivers/Kconfig
> index db94393f47a6..20050e9bb8b3 100644
> --- a/xen/drivers/Kconfig
> +++ b/xen/drivers/Kconfig
> @@ -15,4 +15,8 @@ source "drivers/video/Kconfig"
>  config HAS_VPCI
>  	bool
>  
> +config HAS_VPCI_GUEST_SUPPORT
> +	bool
> +	select HAS_VPCI
> +
>  endmenu
> diff --git a/xen/drivers/vpci/vpci.c b/xen/drivers/vpci/vpci.c
> index 97e115dc5798..23722634d50b 100644
> --- a/xen/drivers/vpci/vpci.c
> +++ b/xen/drivers/vpci/vpci.c
> @@ -40,6 +40,49 @@ extern vpci_register_init_t *const __start_vpci_array[];
>  extern vpci_register_init_t *const __end_vpci_array[];
>  #define NUM_VPCI_INIT (__end_vpci_array - __start_vpci_array)
>  
> +#ifdef CONFIG_HAS_VPCI_GUEST_SUPPORT
> +static int add_virtual_device(struct pci_dev *pdev)

This seems quite generic, IMO it would better named
`assign_{guest,virtual}_sbdf()` or similar, unless there are plans to
add more code here that's not strictly only about setting the guest
SBDF.

> +{
> +    struct domain *d = pdev->domain;
> +    unsigned int new_dev_number;
> +
> +    if ( is_hardware_domain(d) )
> +        return 0;
> +
> +    ASSERT(rw_is_write_locked(&pdev->domain->pci_lock));

Shouldn't the assert be done before the is_hardware_domain() check, so
that we assert that all possible paths (even those from dom0) have
taken the correct lock?

> +
> +    /*
> +     * Each PCI bus supports 32 devices/slots at max or up to 256 when
> +     * there are multi-function ones which are not yet supported.
> +     */
> +    if ( pdev->sbdf.fn )
> +    {
> +        gdprintk(XENLOG_ERR, "%pp: only function 0 passthrough supported\n",
> +                 &pdev->sbdf);
> +        return -EOPNOTSUPP;
> +    }
> +    new_dev_number = find_first_zero_bit(d->vpci_dev_assigned_map,
> +                                         VPCI_MAX_VIRT_DEV);
> +    if ( new_dev_number == VPCI_MAX_VIRT_DEV )
> +        return -ENOSPC;
> +
> +    __set_bit(new_dev_number, &d->vpci_dev_assigned_map);
> +
> +    /*
> +     * Both segment and bus number are 0:
> +     *  - we emulate a single host bridge for the guest, e.g. segment 0
> +     *  - with bus 0 the virtual devices are seen as embedded
> +     *    endpoints behind the root complex
> +     *
> +     * TODO: add support for multi-function devices.
> +     */
> +    pdev->vpci->guest_sbdf = PCI_SBDF(0, 0, new_dev_number, 0);
> +
> +    return 0;
> +}
> +
> +#endif /* CONFIG_HAS_VPCI_GUEST_SUPPORT */
> +
>  void vpci_deassign_device(struct pci_dev *pdev)
>  {
>      unsigned int i;
> @@ -49,6 +92,12 @@ void vpci_deassign_device(struct pci_dev *pdev)
>      if ( !has_vpci(pdev->domain) || !pdev->vpci )
>          return;
>  
> +#ifdef CONFIG_HAS_VPCI_GUEST_SUPPORT
> +    if ( pdev->vpci->guest_sbdf.sbdf != ~0 )
> +        __clear_bit(pdev->vpci->guest_sbdf.dev,
> +                    &pdev->domain->vpci_dev_assigned_map);
> +#endif
> +
>      spin_lock(&pdev->vpci->lock);
>      while ( !list_empty(&pdev->vpci->handlers) )
>      {
> @@ -103,6 +152,13 @@ int vpci_assign_device(struct pci_dev *pdev)
>      INIT_LIST_HEAD(&pdev->vpci->handlers);
>      spin_lock_init(&pdev->vpci->lock);
>  
> +#ifdef CONFIG_HAS_VPCI_GUEST_SUPPORT
> +    pdev->vpci->guest_sbdf.sbdf = ~0;

I think ~0 wants to be in a define here:

#define INVALID_GUEST_SBDF ~0

Or similar.

Thanks, Roger.