Re: [Qemu-devel] [PATCH 03/10] spapr: add option vector handling in CAS-generated resets

[David Gibson]

On Mon, Oct 24, 2016 at 11:47:29PM -0500, Michael Roth wrote:
> In some cases, ibm,client-architecture-support calls can fail. This
> could happen in the current code for situations where the modified
> device tree segment exceeds the buffer size provided by the guest
> via the call parameters. In these cases, QEMU will reset, allowing
> an opportunity to regenerate the device tree from scratch via
> boot-time handling. There are potentially other scenarios as well,
> not currently reachable in the current code, but possible in theory,
> such as cases where device-tree properties or nodes need to be removed.
> 
> We currently don't handle either of these properly for option vector
> capabilities however. Instead of carrying the negotiated capability
> beyond the reset and creating the boot-time device tree accordingly,
> we start from scratch, generating the same boot-time device tree as we
> did prior to the CAS-generated and the same device tree updates as we
> did before. This could (in theory) cause us to get stuck in a reset
> loop. This hasn't been observed, but depending on the extensiveness
> of CAS-induced device tree updates in the future, could eventually
> become an issue.
> 
> Address this by pulling capability-related device tree
> updates resulting from CAS calls into a common routine,
> spapr_dt_cas_updates(), and adding an sPAPROptionVector*
> parameter that allows us to test for newly-negotiated capabilities.
> We invoke it as follows:
> 
> 1) When ibm,client-architecture-support gets called, we
>    call spapr_dt_cas_updates() with the set of capabilities
>    added since the previous call to ibm,client-architecture-support.
>    For the initial boot, or a system reset generated by something
>    other than the CAS call itself, this set will consist of *all*
>    options supported both the platform and the guest. For calls
>    to ibm,client-architecture-support immediately after a CAS-induced
>    reset, we call spapr_dt_cas_updates() with only the set
>    of capabilities added since the previous call, since the other
>    capabilities will have already been addressed by the boot-time
>    device-tree this time around. In the unlikely event that
>    capabilities are *removed* since the previous CAS, we will
>    generate a CAS-induced reset. In the unlikely event that we
>    cannot fit the device-tree updates into the buffer provided
>    by the guest, well generate a CAS-induced reset.
> 
> 2) When a CAS update results in the need to reset the machine and
>    include the updates in the boot-time device tree, we call the
>    spapr_dt_cas_updates() using the full set of negotiated
>    capabilities as part of the reset path. At initial boot, or after
>    a reset generated by something other than the CAS call itself,
>    this set will be empty, resulting in what should be the same
>    boot-time device-tree as we generated prior to this patch. For
>    CAS-induced reset, this routine will be called with the full set of
>    capabilities negotiated by the platform/guest in the previous
>    CAS call, which should result in CAS updates from previous call
>    being accounted for in the initial boot-time device tree.
> 
> Signed-off-by: Michael Roth <address@hidden>
> Reviewed-by: David Gibson <address@hidden>

One little nit..

[snip]
> @@ -1013,13 +1013,27 @@ static target_ulong 
> h_client_architecture_support(PowerPCCPU *cpu_,
>       * of guest input. To model these properly we'd want some sort of mask,
>       * but since they only currently apply to memory migration as defined
>       * by LoPAPR 1.1, 14.5.4.8, which QEMU doesn't implement, we don't need
> -     * to worry about this.
> +     * to worry about this for now.
>       */
> +    ov5_cas_old = spapr_ovec_clone(spapr->ov5_cas);
> +    /* full range of negotiated ov5 capabilities */
>      spapr_ovec_intersect(spapr->ov5_cas, spapr->ov5, ov5_guest);
>      spapr_ovec_cleanup(ov5_guest);
> +    /* capabilities that have been added since CAS-generated guest reset.
> +     * if capabilities have since been removed, generate another reset
> +     */
> +    ov5_updates = spapr_ovec_new();
> +    spapr->cas_reboot = spapr_ovec_diff(ov5_updates,
> +                                        ov5_cas_old, spapr->ov5_cas);
> +
> +    if (!spapr->cas_reboot) {
> +        spapr->cas_reboot =
> +            spapr_h_cas_compose_response(spapr, args[1], args[2], cpu_update,
> +                                         ov5_updates);

spapr->cas_reboot is a bool, whereas spapr_h_cas_compose_response()
returns an int error code.  Now that C has real bools, I think the
compiler will do the right thing here, but I'd prefer to see an explicit

cas_reboot = (spapr_h_cas_compose_response() != 0)

for clarity.

> +    }
> +    spapr_ovec_cleanup(ov5_updates);
>  
> -    if (spapr_h_cas_compose_response(spapr, args[1], args[2],
> -                                     cpu_update)) {
> +    if (spapr->cas_reboot) {
>          qemu_system_reset_request();
>      }
>  
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index a37eee8..b6f9f1b 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -75,6 +75,7 @@ struct sPAPRMachineState {
>      bool has_graphics;
>      sPAPROptionVector *ov5;         /* QEMU-supported option vectors */
>      sPAPROptionVector *ov5_cas;     /* negotiated (via CAS) option vectors */
> +    bool cas_reboot;
>  
>      uint32_t check_exception_irq;
>      Notifier epow_notifier;
> @@ -586,7 +587,8 @@ void spapr_events_init(sPAPRMachineState *sm);
>  void spapr_dt_events(void *fdt, uint32_t check_exception_irq);
>  int spapr_h_cas_compose_response(sPAPRMachineState *sm,
>                                   target_ulong addr, target_ulong size,
> -                                 bool cpu_update);
> +                                 bool cpu_update,
> +                                 sPAPROptionVector *ov5_updates);
>  sPAPRTCETable *spapr_tce_new_table(DeviceState *owner, uint32_t liobn);
>  void spapr_tce_table_enable(sPAPRTCETable *tcet,
>                              uint32_t page_shift, uint64_t bus_offset,

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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