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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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;