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

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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;
+
     return (has_hvm_container_vcpu(v) ? hvm_local_events_need_delivery(v) :
             (vcpu_info(v, evtchn_upcall_pending) &&
              !vcpu_info(v, evtchn_upcall_mask)));

---

But seeing your comments I now wonder whether that's appropriate. Is there 
anyway in Xen to know whether Xen is in hypercall context or not?

Another way to fix this would be to change both {hap/sh}_set_allocation 
functions to only call hypercall_check_preempt if current != idle_domain, 
and in the idle domain case just call softirq_pending (which is the same 
that the above change achieves).

Roger.

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