Re: [PATCH v2 1/1] vpci: Add resizable bar support

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

On Mon, Dec 09, 2024 at 02:59:31PM +0100, Jan Beulich wrote:
> On 02.12.2024 07:09, Jiqian Chen wrote:
> > +static int cf_check init_rebar(struct pci_dev *pdev)
> > +{
> > +    uint32_t ctrl;
> > +    unsigned int rebar_offset, nbars;
> > +
> > +    rebar_offset = pci_find_ext_capability(pdev->sbdf, PCI_EXT_CAP_ID_REBAR);
> > +
> > +    if ( !rebar_offset )
> > +        return 0;
> > +
> > +    ctrl = pci_conf_read32(pdev->sbdf, rebar_offset + PCI_REBAR_CTRL);
> > +    nbars = MASK_EXTR(ctrl, PCI_REBAR_CTRL_NBAR_MASK);
> > +
> > +    for ( unsigned int i = 0; i < nbars; i++, rebar_offset += PCI_REBAR_CTRL )
> > +    {
> > +        int rc;
> > +
> > +        rc = vpci_add_register(pdev->vpci, vpci_hw_read32, vpci_hw_write32,
> > +                               rebar_offset + PCI_REBAR_CAP, 4, NULL);
> 
> The capability register is r/o aiui. While permitting hwdom to write it is
> fine, DomU-s shouldn't be permitted doing so, just in case. (An alternative
> to making handler selection conditional here would be to bail early for the
> !hwdom case, accompanied by a TODO comment. This would then also address
> the lack of virtualization of the extended capability chain, as we may not
> blindly expose all capabilities to DomU-s.)

I don't think we can safely expose this capability to domUs by
default, so my preference would be a returning an error in that case
(and printing a log message indicating ReBAR is not supported for
domUs).

Note it's already not exposed to domUs by not being part of
supported_caps in init_header().

Regards, Roger