Re: [Xen-users] Debugging DomU

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

(Reinstating the list, please keep cc's)

On Tue, 2015-05-05 at 21:57 +0000, Chris Brand wrote:
> Hi Ian,
> 
> Thanks for the help!
> 
> I think that you're right about a very early abort.
> 
> I've been trying to do what you suggested. The problem is that
> tools/libxc is part of libxenguest, but most (all ?) those functions
> that your ma_zero_hack() calls are in Xen proper. I guess that there's
> a quite a lot of difference between the way Dom0 is brought and up and
> the way DomU is. I'm going to keep plugging away at it, but thought it
> was worth mentioning just in case you have one of those "oh, that's
> easy. All you need to do is..." reactions :-)

ma_zero_hack was more for a vague reference than anything.

alloc_magic_pages is an OK reference for this from the domain builder
side.

From that the userspace equivalent of p2m_populate_ram is
xc_domain_populate_physmap_exact and the call to xc_clear_domain_page
encapsulates the equivalents of gvirt_to_maddr and (un)map_domain_page,
which is essentially xc_map_foreign_range+munmap+xc_domain_cacheflush,
but you don't want that memset as is of course.

Ian.

> 
> Thanks again for the help so far,
> 
> Chris
> 
> -----Original Message-----
> From: Ian Campbell [mailto:ian.campbell@xxxxxxxxxx] 
> Sent: Saturday, 02 May, 2015 2:00 AM
> To: Chris Brand
> Cc: xen-users@xxxxxxxxxxxxx
> Subject: Re: [Xen-users] Debugging DomU
> 
> On Fri, 2015-05-01 at 19:16 +0000, Chris Brand wrote:
> > > I think xenctx (from $prefix/lib/xen/bin) should work, which will get you 
> > > at least register state including, hopefully, a useful r15 which gdb or 
> > > addr2line can convert back to a source line in the kernel.
> > 
> > Thanks, Ian. That does give me something to go on, at least. One thing that 
> > leaps out is "PC = 0000000c".
> 
> That usually suggests that the kernel has taken an abort before the vector 
> table has been initialised...
> 
> You might find that the LR register for the current mode gives you the PC 
> where the fault occurred, but often these things are recursive since there is 
> nothing mapped at address 0 so it just keeps on faulting which clobbers the 
> LR repeatedly.
> 
> For dom0 I have in the past used the patch below to turn such events into 
> something more useful, you could try applying something similar to the 
> userspace domain builder. Perhaps bodge something into 
> tools/libxc/xc_dom_arm.c e.g. in alloc_magic_pages or maybe 
> arch_setup_bootlate?
> 
> If you do get something like that working then posting it (maybe to
> -devel) so people can pull it from the archives next time (since I would 
> assume it would be too ugly to actually apply) would be very useful. Be sure 
> to say "Potentially useful debug hack, not to be applied" to warn off 
> potential reviewers ;-)
> 
> Ian.
> 
> commit 731d32f462353635deeddc24ae530acd5702ec80
> Author: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Date:   Fri Dec 6 10:00:58 2013 +0000
> 
>     DEBUG-HACK: Add hypervisor traps to page at address 0
>     
>     This catches any early guest traps and redirects them to the hypervisor.
> 
> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c index 
> e501d2a..5fefdab 100644
> --- a/xen/arch/arm/domain_build.c
> +++ b/xen/arch/arm/domain_build.c
> @@ -958,6 +958,34 @@ static void initrd_load(struct kernel_info *kinfo)
>          unmap_domain_page(dst);
>          offs += l;
>      }
> +
> +static void ma_zero_hack(struct domain *d) {
> +    paddr_t ma = 0;
> +    uint32_t *dst;
> +    int res;
> +//    return;
> +
> +    /*
> +     * Set temporary guest traps with 0xe14fff7c which is hvc(0xfffc) a hyp
> +     * panic! TODO: Find why:
> +     *  1) Xen abort directly after local_abort_enable when
> +     *  the p2m_populate_ram is not here.
> +     *  2) Linux doesn't start without this trick
> +     */
> +    p2m_populate_ram(d, 0x0, 0x1000 - 1);
> +
> +    res = gvirt_to_maddr(0, &ma);
> +    if ( res )
> +        panic(XENLOG_ERR "Unable to translate guest address\n");
> +
> +    printk("MA0 hack MFN at %"PRIpaddr"\n", ma);
> +
> +    dst = map_domain_page(ma >> PAGE_SHIFT);
> +//    dst[0] = dst[1] = dst[2] = 0xe14fff7f;
> +    dst[0] = dst[1] = dst[2] = 0xe14fff7c;
> +    dst[3] = dst[4] = dst[5] = 0xe14fff7c;
> +    unmap_domain_page(dst);
>  }
>  
>  int construct_dom0(struct domain *d)
> @@ -1015,6 +1043,8 @@ int construct_dom0(struct domain *d)
>      initrd_load(&kinfo);
>      dtb_load(&kinfo);
>  
> +    ma_zero_hack(d);
> +
>      discard_initial_modules();
>  
>      v->is_initialised = 1;
> 
> 



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