Re: [Xen-devel] [PATCH] xen/arm: p2m: Restrict preemption check in apply_p2m_changes

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, May 05, 2015 at 04:02:09PM +0100, Julien Grall wrote:
> The commit 569fb6c "xen/arm: Data abort exception (R/W) mem_access
> events" makes apply_p2m_changes to call hypercall_preempt_check for any
> operation rather than for relinquish.
> 
> The function hypercall_preempt_check call local_events_need_delivery
> which rely on the current VCPU is not an idle VCPU.
> Although, during DOM0 building the current VCPU is an idle one. This
> would make Xen crash with the following stack trace:
> 
> (XEN) CPU0: Unexpected Trap: Data Abort
> [...]
> (XEN) Xen call trace:
> (XEN)    [<00256ef4>] apply_p2m_changes+0x210/0x1190 (PC)
> (XEN)    [<002506b4>] gic_events_need_delivery+0x5c/0x13c (LR)
> (XEN)    [<002580ec>] map_mmio_regions+0x64/0x74
> (XEN)    [<00251958>] gicv2v_setup+0xf8/0x150
> (XEN)    [<00250964>] gicv_setup+0x20/0x30
> (XEN)    [<0024cb3c>] arch_domain_create+0x170/0x244
> (XEN)    [<00207df0>] domain_create+0x2ac/0x4d8
> (XEN)    [<0028e3d0>] start_xen+0xcbc/0xee4
> (XEN)    [<00200540>] paging+0x94/0xd8
> (XEN)
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 0:
> (XEN) CPU0: Unexpected Trap: Data Abort
> (XEN)
> (XEN) ****************************************
> 
> hypercall_preempt_check is expecting to be call only when the current
> VCPU belong to a real domain (see x86 behavior).
> 
> As the bug prevents Xen booting on some platform, fix it by only check
> preemption when the current VCPU is an idle one for now. We could
> improve it later.
> 
> Reported-by: Riku Voipio <riku.voipio@xxxxxxxxxx>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> CC: Tamas K Lengyel <tklengyel@xxxxxxxxxxxxx>
> 
> ---
> 
> This bug has been catched during boot on Mustang. This is because we
> have to map large chunk of PCI memory region.
> 
> I was able to reproduce the bug on midway by lowering down
> preempt_count_limit to 16 in apply_p2m_changes.
> 
> Note: This patch superseeds "Make local_events_need_delivery working with idle
> VPCU"
> ---
>  xen/arch/arm/p2m.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 65efa94..59dd23a 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -948,6 +948,7 @@ static int apply_p2m_changes(struct domain *d,
>      const unsigned long sgfn = paddr_to_pfn(start_gpaddr),
>                          egfn = paddr_to_pfn(end_gpaddr);
>      const unsigned int preempt_count_limit = (op == MEMACCESS) ? 1 : 0x2000;
> +    const bool_t preempt = !is_idle_vcpu(current);
>      bool_t flush = false;
>      bool_t flush_pt;
>  
> @@ -980,7 +981,8 @@ static int apply_p2m_changes(struct domain *d,
>           * always make at least one pass as long as preempt_count_limit is
>           * initialized with a value >= 1.
>           */
> -        if ( count >= preempt_count_limit && hypercall_preempt_check() )
> +        if ( preempt && count >= preempt_count_limit
> +             && hypercall_preempt_check() )

Could you  use the softirq_pending() check to deal when there are no domains?

>          {
>              switch ( op )
>              {
> -- 
> 2.1.4
> 
> 
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