Re: [Xen-devel] [PATCH RFC 01/12] x86/paging: introduce paging_set_allocation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.08.16 at 18:00, <roger.pau@xxxxxxxxxx> wrote:
> On Wed, Aug 03, 2016 at 09:37:41AM -0600, Jan Beulich wrote:
>> >>> On 03.08.16 at 17:28, <george.dunlap@xxxxxxxxxx> wrote:
>> > On 03/08/16 16:25, Jan Beulich wrote:
>> >>>>> On 03.08.16 at 17:11, <George.Dunlap@xxxxxxxxxxxxx> wrote:
>> >>> On Tue, Aug 2, 2016 at 5:12 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >>>>>>> On 02.08.16 at 17:49, <roger.pau@xxxxxxxxxx> wrote:
>> >>>>> On Tue, Aug 02, 2016 at 11:47:22AM +0200, Roger Pau Monne wrote:
>> >>>>>> On Fri, Jul 29, 2016 at 05:47:24PM +0100, Andrew Cooper wrote:
>> >>>>>>> As this is for the construction of dom0, it would be better to take a
>> >>>>>>> preemptible pointer, loop in construct_dom0(), with a
>> >>>>>>> process_pending_softirqs() call in between.
>> >>>>>>
>> >>>>>> Now fixed.
>> >>>>>
>> >>>>> Hm, I have to stand corrected, using hypercall_preempt_check (as
>> >>>>> any of the *_set_allocation function use), is not safe at this point:
>> >>>>>
>> >>>>> (XEN) ----[ Xen-4.8-unstable  x86_64  debug=y  Tainted:    C  ]----
>> >>>>> (XEN) CPU:    0
>> >>>>> (XEN) RIP:    e008:[<ffff82d08022fd47>] 
>> >>> hap.c#local_events_need_delivery+0x27/0x40
>> >>>>> (XEN) RFLAGS: 0000000000010246   CONTEXT: hypervisor
>> >>>>> (XEN) rax: 0000000000000000   rbx: ffff83023f5a5000   rcx: 
>> >>>>> ffff82d080312900
>> >>>>> (XEN) rdx: 0000000000000001   rsi: ffff83023f5a56c8   rdi: 
>> >>>>> ffff8300b213d000
>> >>>>> (XEN) rbp: ffff82d080307cc8   rsp: ffff82d080307cc8   r8:  
>> >>>>> 0180000000000000
>> >>>>> (XEN) r9:  0000000000000000   r10: 0000000000247000   r11: 
>> >>>>> ffff82d08029a5b0
>> >>>>> (XEN) r12: 0000000000000011   r13: 00000000000023ac   r14: 
>> >>>>> ffff82d080307d4c
>> >>>>> (XEN) r15: ffff83023f5a56c8   cr0: 000000008005003b   cr4: 
>> >>>>> 00000000001526e0
>> >>>>> (XEN) cr3: 00000000b20fc000   cr2: 0000000000000000
>> >>>>> (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: e008
>> >>>>> (XEN) Xen code around <ffff82d08022fd47> 
>> >>> (hap.c#local_events_need_delivery+0x27/0x40):
>> >>>>> (XEN)  0d ad fa ff 48 8b 47 08 <80> 38 00 74 09 80 78 01 00 0f 94 c0 
>> >>>>> eb 02 31 
> 
>> >>> c0
>> >>>>> (XEN) Xen stack trace from rsp=ffff82d080307cc8:
>> >>>>> (XEN)    ffff82d080307d08 ffff82d08022fc47 0000000000000000 
> ffff83023f5a5000
>> >>>>> (XEN)    ffff83023f5a5648 0000000000000000 ffff82d080307d4c 
> 0000000000002400
>> >>>>> (XEN)    ffff82d080307d38 ffff82d08020008c 00000000000ffffd 
> ffff8300b1efd000
>> >>>>> (XEN)    ffff83023f5a5000 ffff82d080307d4c ffff82d080307d78 
> ffff82d0802cad30
>> >>>>> (XEN)    0000000000203000 ffff83023f5a5000 ffff82d0802bf860 
> 0000000000000000
>> >>>>> (XEN)    0000000000000001 ffff83000008bef0 ffff82d080307de8 
> ffff82d0802c91e0
>> >>>>> (XEN)    ffff82d080307de8 ffff82d080143900 ffff82d080307de8 
> 0000000000000000
>> >>>>> (XEN)    ffff83000008bf00 ffff82d0802eb480 ffff82d080307dc4 
> ffff82d08028b1cd
>> >>>>> (XEN)    ffff83000008bf00 0000000000000000 0000000000000001 
> ffff83023f5a5000
>> >>>>> (XEN)    ffff82d080307f08 ffff82d0802bf0c9 0000000000000000 
> 0000000000000000
>> >>>>> (XEN)    0000000000000000 ffff82d080307f18 ffff83000008bee0 
> 0000000000000001
>> >>>>> (XEN)    0000000000000001 0000000000000001 0000000000000000 
> 0000000000100000
>> >>>>> (XEN)    0000000000000001 0000000000247000 ffff83000008bef4 
> 0000000000100000
>> >>>>> (XEN)    ffff830100000000 0000000000247001 0000000000000014 
> 0000000100000000
>> >>>>> (XEN)    ffff8300ffffffec ffff83000008bef0 ffff82d0802e0640 
> ffff83000008bfb0
>> >>>>> (XEN)    0000000000000000 0000000000000000 0000000000000111 
> 0000000800000000
>> >>>>> (XEN)    000000010000006e 0000000000000003 00000000000002f8 
> 0000000000000000
>> >>>>> (XEN)    00000000ad5c0bd0 0000000000000000 0000000000000001 
> 0000000000000008
>> >>>>> (XEN)    0000000000000000 ffff82d080100073 0000000000000000 
> 0000000000000000
>> >>>>> (XEN)    0000000000000000 0000000000000000 0000000000000000 
> 0000000000000000
>> >>>>> (XEN) Xen call trace:
>> >>>>> (XEN)    [<ffff82d08022fd47>] 
>> >>>>> hap.c#local_events_need_delivery+0x27/0x40
>> >>>>> (XEN)    [<ffff82d08022fc47>] hap_set_allocation+0x107/0x130
>> >>>>> (XEN)    [<ffff82d08020008c>] paging_set_allocation+0x4c/0x80
>> >>>>> (XEN)    [<ffff82d0802cad30>] domain_build.c#hvm_setup_p2m+0x70/0x1a0
>> >>>>> (XEN)    [<ffff82d0802c91e0>] 
>> >>>>> domain_build.c#construct_dom0_hvm+0x60/0x120
>> >>>>> (XEN)    [<ffff82d0802bf0c9>] __start_xen+0x1ea9/0x23a0
>> >>>>> (XEN)    [<ffff82d080100073>] __high_start+0x53/0x60
>> >>>>> (XEN)
>> >>>>> (XEN) Pagetable walk from 0000000000000000:
>> >>>>
>> >>>> Sadly you don't make clear what pointer it is that is NULL at that 
>> >>>> point.
>> >>>
>> >>> It sounds from what he says in the following paragraph like current is 
> NULL.
>> >> 
>> >> I don't recall us re-setting current to this late in the boot process.
>> >> Even during early boot we set it to a bogus non-NULL value rather
>> >> than NULL.
>> >> 
>> >>>>> I've tried setting current to d->vcpu[0], but that just makes the call 
>> >>>>> to
>> >>>>> hypercall_preempt_check crash in some scheduler assert. In any case, 
>> >>>>> I've
>> >>>>> added the preempt parameter to the paging_set_allocation function, but 
>> >>>>> I
>> >>>>> don't plan to use it in the domain builder for the time being. Does 
>> >>>>> that
>> >>>>> sound right?
>> >>>>
>> >>>> Not really, new huge latency issues like this shouldn't be reintroduced;
>> >>>> we've been fighting hard to get rid of those (and we still occasionally
>> >>>> find some no-one had noticed before).
>> >>>
>> >>> You mean latency in processing softirqs?
>> >>>
>> >>> Maybe what we need to do is to make local_events_need_delivery() safe
>> >>> to call at this point by having it return 0 if current is NULL rather
>> >>> than crashing?
>> >> 
>> >> That would have the same effect - no softirq processing, and hence
>> >> possible time issues on huge systems.
>> > 
>> > No, local_events_delivery() only checks to see if the current vcpu has
>> > outstanding virtual interrupts.  The other half of
>> > hypercall_preempt_check() checks for softirqs, which doesn't appear to
>> > rely on having current available.
>> 
>> Good point, but
>> - current should nevertheless not be NULL (afaict at least),
>> - hypercall_preempt_check() is probably the wrong construct,
>>   as we're no in a hypercall.
>> The latter of course could be addressed by, as you did suggest,
>> some refinement to one of the pieces it's being made up from,
>> but I'm not sure that would be better than perhaps making its
>> invocation conditional (with some better alternative in the "else"
>> case) in hap_set_allocation(). Not the least because any
>> adjustment to hypercall_preempt_check() itself would affect all
>> other of its users.
> 
> I had added the following patch to my queue in order to fix this:
> 
> ---
> xen/x86: allow calling hypercall_preempt_check with the idle domain
> 
> This allows using hypercall_preempt_check while building Dom0.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> diff --git a/xen/include/asm-x86/event.h b/xen/include/asm-x86/event.h
> index a82062e..d55a8bd 100644
> --- a/xen/include/asm-x86/event.h
> +++ b/xen/include/asm-x86/event.h
> @@ -23,6 +23,10 @@ int hvm_local_events_need_delivery(struct vcpu *v);
>  static inline int local_events_need_delivery(void)
>  {
>      struct vcpu *v = current;
> +
> +    if ( is_idle_vcpu(v) )
> +        return 0;

As said, I think it would be better to not add it here, unless there
is a significant amount of other calls into here from idle vCPU-s
with your changes.

Jan

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