[Xen-devel] Re: Xen paravirt frontend block hang

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Christopher S. Aker wrote:
> Sorry for the noise if this isn't the appropriate venue for this.  I 
> posted this last month to xen-devel:
> http://lists.xensource.com/archives/html/xen-devel/2007-11/msg00777.html
>
> I can reliably cause a paravirt_ops Xen guest to hang during intensive 
> IO.  My current recipe is an untar/tar loop, without compression, of a 
> kernel tree.  For example:
> wget http://kernel.org/pub/linux/kernel/v2.6/linux-2.6.23.tar.bz2
> bzip2 -d linux-2.6.23.tar.bz2
>
> while true;
>   echo `date`
>   tar xf linux-2.6.23.tar
>   tar cf linux-2.6.23.tar linux-2.6.23
> done
>
> After a few loops, anything that touches the xvd device that hung will 
> get stuck in D state.
I've been running this all night without seeing any problem.  I'm using 
current x86.git#testing with a few local patches, but nothing especially 
relevent-looking.
Could you try the attached patch to see if it makes any difference?

   J

> This happens on both a 2.6.16 and 2.6.18 dom0 (3.1.2 tools).  Paravirt 
> guests I've tried that exhibit the problem: 2.6.23.8, 2.6.23.12, and 
> 2.6.24-rc6.  It does *not* occur using the Xensource 2.6.18 domU tree 
> from 3.1.2.  In all cases, the host continues to run fine, nothing out 
> of the ordinary is logged on the dom0 side, xenstore reports the 
> status of the devices is fine.
> Can anyone reproduce this problem, or let me know what else I can 
> provide to help track this down?
> Thanks,
> -Chris
> _______________________________________________
> Virtualization mailing list
> Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> https://lists.linux-foundation.org/mailman/listinfo/virtualization

Subject: xen: use iret instruction all the time

Change iret implementation to not be dependent on direct-access vcpu
structure.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>

---
 arch/x86/xen/enlighten.c |    3 +--
 arch/x86/xen/xen-asm.S   |   11 +++--------
 arch/x86/xen/xen-ops.h   |    2 +-
 3 files changed, 5 insertions(+), 11 deletions(-)

===================================================================
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -860,7 +860,6 @@ void __init xen_setup_vcpu_info_placemen
                pv_irq_ops.irq_disable = xen_irq_disable_direct;
                pv_irq_ops.irq_enable = xen_irq_enable_direct;
                pv_mmu_ops.read_cr2 = xen_read_cr2_direct;
-               pv_cpu_ops.iret = xen_iret_direct;
        }
 }
 
@@ -964,7 +963,7 @@ static const struct pv_cpu_ops xen_cpu_o
        .read_tsc = native_read_tsc,
        .read_pmc = native_read_pmc,
 
-       .iret = (void *)&hypercall_page[__HYPERVISOR_iret],
+       .iret = xen_iret,
        .irq_enable_syscall_ret = NULL,  /* never called */
 
        .load_tr_desc = paravirt_nop,
===================================================================
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -130,13 +130,8 @@ ENDPATCH(xen_restore_fl_direct)
        current stack state in whatever form its in, we keep things
        simple by only using a single register which is pushed/popped
        on the stack.
-
-       Non-direct iret could be done in the same way, but it would
-       require an annoying amount of code duplication.  We'll assume
-       that direct mode will be the common case once the hypervisor
-       support becomes commonplace.
  */
-ENTRY(xen_iret_direct)
+ENTRY(xen_iret)
        /* test eflags for special cases */
        testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
        jnz hyper_iret
@@ -150,9 +145,9 @@ ENTRY(xen_iret_direct)
        GET_THREAD_INFO(%eax)
        movl TI_cpu(%eax),%eax
        movl __per_cpu_offset(,%eax,4),%eax
-       lea per_cpu__xen_vcpu_info(%eax),%eax
+       mov per_cpu__xen_vcpu(%eax),%eax
 #else
-       movl $per_cpu__xen_vcpu_info, %eax
+       movl per_cpu__xen_vcpu, %eax
 #endif
 
        /* check IF state we're restoring */
===================================================================
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -63,5 +63,5 @@ DECL_ASM(unsigned long, xen_save_fl_dire
 DECL_ASM(unsigned long, xen_save_fl_direct, void);
 DECL_ASM(void, xen_restore_fl_direct, unsigned long);
 
-void xen_iret_direct(void);
+void xen_iret(void);
 #endif /* XEN_OPS_H */

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