Re: [PATCH 07/12] libxl: Allow stubdomain to control interupts of PCI device

[Anthony PERARD <anthony.perard@vates.tech>]

On Thu, May 16, 2024 at 03:58:28PM +0200, Marek Marczykowski-Górecki wrote:
> Especially allow it to control MSI/MSI-X enabling bits. This part only
> writes a flag to a sysfs, the actual implementation is on the kernel
> side.
>
> This requires Linux >= 5.10 in dom0 (or relevant patch backported).

Does it not work before 5.10? Because the
Documentation/ABI/testing/sysfs-driver-pciback in linux tree say that
allow_interrupt_control is in 5.6.

> diff --git a/tools/libs/light/libxl_pci.c b/tools/libs/light/libxl_pci.c
> index 96cb4da0794e..6f357b70b815 100644
> --- a/tools/libs/light/libxl_pci.c
> +++ b/tools/libs/light/libxl_pci.c
> @@ -1513,6 +1513,14 @@ static void pci_add_dm_done(libxl__egc *egc,
>              rc = ERROR_FAIL;
>              goto out;
>          }
> +    } else if (libxl_is_stubdom(ctx, domid, NULL)) {
> +        /* Allow acces to MSI enable flag in PCI config space for the stubdom */

s/acces/access/

> +        if ( sysfs_write_bdf(gc, SYSFS_PCIBACK_DRIVER"/allow_interrupt_control",
> +                             pci) < 0 ) {
> +            LOGD(ERROR, domainid, "Setting allow_interrupt_control for device");
> +            rc = ERROR_FAIL;
> +            goto out;

Is it possible to make this non-fatal for cases where the kernel is
older than the introduction of the new setting? Or does pci passthrough
doesn't work at all with a stubdom before the change in the kernel?

If making this new setting conditional is an option, you could
potentially improve the error code returned by sysfs_write_bdf() to
distinguish between an open() failure and write() failure, to avoid
checking the existance of the path ahead of the call. But maybe that
pointless because it doesn't appear possible to distinguish between
permission denied and not found.

Thanks,

--

Anthony Perard | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech