Re: [linux-linus test] 183794: regressions - FAIL

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 22 Nov 2023, Juergen Gross wrote:
> On 22.11.23 04:07, Stefano Stabellini wrote:
> > On Mon, 20 Nov 2023, Stefano Stabellini wrote:
> > > On Mon, 20 Nov 2023, Juergen Gross wrote:
> > > > On 20.11.23 03:21, osstest service owner wrote:
> > > > > flight 183794 linux-linus real [real]
> > > > > http://logs.test-lab.xenproject.org/osstest/logs/183794/
> > > > > 
> > > > > Regressions :-(
> > > > > 
> > > > > Tests which did not succeed and are blocking,
> > > > > including tests which could not be run:
> > > > >    test-arm64-arm64-examine      8 reboot                   fail REGR.
> > > > > vs.
> > > > > 183766
> > > > 
> > > > I'm seeing the following in the serial log:
> > > > 
> > > > Nov 20 00:25:41.586712 [    0.567318] kernel BUG at
> > > > arch/arm64/xen/../../arm/xen/enlighten.c:164!
> > > > Nov 20 00:25:41.598711 [    0.574002] Internal error: Oops - BUG:
> > > > 00000000f2000800 [#1] PREEMPT SMP
> > > > 
> > > > The related source code lines in the kernel are:
> > > > 
> > > > ········err = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info,
> > > > xen_vcpu_nr(cpu),
> > > > ································ &info);
> > > > ········BUG_ON(err);
> > > > 
> > > > I suspect commit 20f3b8eafe0ba to be the culprit.
> > > > 
> > > > Stefano, could you please have a look?
> > 
> > The good news and bad news is that I cannot repro this neither with nor
> > without CONFIG_UNMAP_KERNEL_AT_EL0. I looked at commit 20f3b8eafe0ba but
> > I cannot see anything wrong with it. Looking at the register dump, from:
> > 
> > x0 : fffffffffffffffa
> > 
> > I am guessing the error was -ENXIO which is returned from map_guest_area
> > in Xen.
> > 
> > Could it be that the struct is crossing a page boundary? Or that it is
> > not 64-bit aligned? Do we need to do something like the following?
> > 
> > diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> > index 9afdc4c4a5dc..5326070c5dc0 100644
> > --- a/arch/arm/xen/enlighten.c
> > +++ b/arch/arm/xen/enlighten.c
> > @@ -484,7 +485,7 @@ static int __init xen_guest_init(void)
> >      * for secondary CPUs as they are brought up.
> >      * For uniformity we use VCPUOP_register_vcpu_info even on cpu0.
> >      */
> > -   xen_vcpu_info = alloc_percpu(struct vcpu_info);
> > +   xen_vcpu_info = __alloc_percpu(struct vcpu_info, PAGE_SIZE);
> >     if (xen_vcpu_info == NULL)
> >             return -ENOMEM;
> >   
> 
> May I suggest to use a smaller alignment? What about:
> 
> 1 << fls(sizeof(struct vcpu_info) - 1)

See below

---
[PATCH] arm/xen: fix xen_vcpu_info allocation alignment

xen_vcpu_info is a percpu area than needs to be mapped by Xen.
Currently, it could cross a page boundary resulting in Xen being unable
to map it:

[    0.567318] kernel BUG at arch/arm64/xen/../../arm/xen/enlighten.c:164!
[    0.574002] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP

Fix the issue by using __alloc_percpu and requesting alignment for the
memory allocation.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>

diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
index 9afdc4c4a5dc..09eb74a07dfc 100644
--- a/arch/arm/xen/enlighten.c
+++ b/arch/arm/xen/enlighten.c
@@ -484,7 +484,8 @@ static int __init xen_guest_init(void)
         * for secondary CPUs as they are brought up.
         * For uniformity we use VCPUOP_register_vcpu_info even on cpu0.
         */
-       xen_vcpu_info = alloc_percpu(struct vcpu_info);
+       xen_vcpu_info = __alloc_percpu(sizeof(struct vcpu_info),
+                                              1 << fls(sizeof(struct 
vcpu_info) - 1));
        if (xen_vcpu_info == NULL)
                return -ENOMEM;