Subject: Oops in guest after ioremap() on ARMv7

From: [email-masked] (David Vrabel)
Date: Thu, 22 Dec 2011 16:38:23 +0000

On 22/12/11 14:49, Catalin Marinas wrote:
> On Thu, Dec 22, 2011 at 12:08:07PM +0000, David Vrabel wrote:
>> When running the linux kernel on the ARMv7 envelope model as a guest
>> under the Xen hypervisor there is a oops (see below for an example of
>> the page translation fault) when trying to access ioremap()'d memory.

The translation tables for userspace seem to be also affected.  The
program repeatedly faults with a translation fault on the same address.
 Putting a cache_flush_all() after the call to handle_mm_fault() in
__do_page_fault() makes userspace work as well.

>> The same kernel works fine when not running under the hypervisor.
>>
>> It's a 3.2.0-rc5+ kernel with the two additional linux-arch-arm
>> branches: arm-arch/vexpress and arm-arch/arm-lpae.
>>
>> Calling flush_cache_all() in flush_cache_vmap() makes it work.  What
>> isn't being correctly flushed?  I see that flush_pmd_entry() and
>> cpu_v7_set_pte_ext() already flush the L1 and L2 translation table
>> entries and I can't think of anything else that would need to be flushed
>> (unless the mapped virtual addresses need to be flushed as well?)
>>
>> The "Barrier Litmus Tests and Cookbook" says that a TLB flush and a
>> branch predictor flush are required after a translation table entry
>> update.  This seems not to be done but adding this didn't seem to help
>> (and using local_flush_tlb_all()) in flush_cache_vmap() didn't help either).
>>
>> I don't see anything in the hypervisor that could be causing this as the
>> fault is occurring at stage 1 and not stage 2 translation.
> 
> Interesting error, I don't have an immediate idea of what might be
> wrong, just some questions.
> 
> What's the value of the VTCR register for this guest? Are the
> translation table walks marked as cacheable? Also, are the page table
> attributes Normal Cacheable in the stage 2 translation? The processor
> chooses the more restrictive attribute between stage 1 and stage 2.

VTCR = 0x80002558 which is: Outer Shareable; Normal memory, outer
write-back write-allocate cacheable; Normal memory, inner write-back,
write-allocate cacheable.

L3 TT entries for stage 2 have the following attributes:
Outer-Shareable; Normal, inner write-back cachable; Normal, outer
write-back cacheable.

These look sensible to me.

David