Re: [PATCH v3 2/7] spapr_pci.c: simplify spapr_pci_unplug_request() function handling

[Greg Kurz <groug@kaod.org>]

On Thu, 11 Feb 2021 19:52:41 -0300
Daniel Henrique Barboza <danielhb413@gmail.com> wrote:

> When hotunplugging a PCI function we'll branch out the logic in two cases,
> function zero and non-zero. If non-zero, we'll call spapr_drc_detach() and
> nothing else. If it's function zero, we'll loop it once between all the
> functions in the slot to call spapr_drc_detach() on them, and afterwards
> we'll do another backwards loop where we'll signal the event to the guest.
> 
> We can simplify this logic. We can ignore all the DRC handling for non-zero
> functions, since we'll end up doing that regardless when unplugging function
> zero. And for function zero, everything can be done in a single loop, since
> tt doesn't matter if we end up marking the function DRCs as unplug pending in
> backwards order or not, as long as we call spapr_drc_detach() before issuing
> the hotunplug event to the guest.
> 
> This will also avoid a possible scenario where the user starts to hotunplug
> the slot, starting with a non-zero function, and then delays/forgets to
> hotunplug function zero afterwards. This would keep the function DRC marked
> as unplug requested indefinitely.
> 

... or until the guest is reset, which will no longer happen with this
patch applied, i.e. breaks the long standing policy that machine reset
causes pending hot-unplug requests to succeed. I don't see an obvious
reason to special case non-zero PCI functions.

> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> ---
>  hw/ppc/spapr_pci.c | 44 ++++++++++++++++----------------------------
>  1 file changed, 16 insertions(+), 28 deletions(-)
> 
> diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> index f1c7479816..1791d98a49 100644
> --- a/hw/ppc/spapr_pci.c
> +++ b/hw/ppc/spapr_pci.c
> @@ -1709,38 +1709,26 @@ static void spapr_pci_unplug_request(HotplugHandler *plug_handler,
>              return;
>          }
>  
> -        /* ensure any other present functions are pending unplug */
> -        if (PCI_FUNC(pdev->devfn) == 0) {
> -            for (i = 1; i < 8; i++) {
> -                func_drc = drc_from_devfn(phb, chassis, PCI_DEVFN(slotnr, i));
> -                func_drck = SPAPR_DR_CONNECTOR_GET_CLASS(func_drc);
> -                state = func_drck->dr_entity_sense(func_drc);
> -                if (state == SPAPR_DR_ENTITY_SENSE_PRESENT
> -                    && !spapr_drc_unplug_requested(func_drc)) {
> -                    /*
> -                     * Attempting to remove function 0 of a multifunction
> -                     * device will will cascade into removing all child
> -                     * functions, even if their unplug weren't requested
> -                     * beforehand.
> -                     */
> -                    spapr_drc_detach(func_drc);
> -                }
> -            }
> +        /*
> +         * The hotunplug itself will occur when unplugging function 0,
> +         * regardless of marking any other functions DRCs as pending
> +         * unplug beforehand (since 02a1536eee33).
> +         */
> +        if (PCI_FUNC(pdev->devfn) != 0) {
> +            return;
>          }
>  
> -        spapr_drc_detach(drc);
> +        for (i = 7; i >= 0; i--) {
> +            func_drc = drc_from_devfn(phb, chassis, PCI_DEVFN(slotnr, i));
> +            func_drck = SPAPR_DR_CONNECTOR_GET_CLASS(func_drc);
> +            state = func_drck->dr_entity_sense(func_drc);
>  
> -        /* if this isn't func 0, defer unplug event. otherwise signal removal
> -         * for all present functions
> -         */
> -        if (PCI_FUNC(pdev->devfn) == 0) {
> -            for (i = 7; i >= 0; i--) {
> -                func_drc = drc_from_devfn(phb, chassis, PCI_DEVFN(slotnr, i));
> -                func_drck = SPAPR_DR_CONNECTOR_GET_CLASS(func_drc);
> -                state = func_drck->dr_entity_sense(func_drc);
> -                if (state == SPAPR_DR_ENTITY_SENSE_PRESENT) {
> -                    spapr_hotplug_req_remove_by_index(func_drc);
> +            if (state == SPAPR_DR_ENTITY_SENSE_PRESENT) {
> +                /* Mark the DRC as requested unplug if needed. */
> +                if (!spapr_drc_unplug_requested(func_drc)) {
> +                    spapr_drc_detach(func_drc);
>                  }
> +                spapr_hotplug_req_remove_by_index(func_drc);
>              }
>          }
>      }