Re: [Xen-devel] [PATCH] xen: arm: invalidate caches after map_domain_page done

[Andrii Tseglytskyi <andrii.tseglytskyi@xxxxxxxxxxxxxxx>]

On Fri, Aug 1, 2014 at 1:28 PM, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
>
>
> On 01/08/14 11:01, Andrii Tseglytskyi wrote:
>>
>> On Fri, Aug 1, 2014 at 12:23 PM, Julien Grall <julien.grall@xxxxxxxxxx>
>> wrote:
>>>
>>>
>>> Hi Andrii,
>>>
>>>
>>> On 01/08/14 08:25, Andrii Tseglytskyi wrote:
>>>>
>>>>
>>>> In some cases, memory page returned by map_domain_page() contains
>>>> invalid data. Issue is observed when map_domain_page() is used
>>>> immediately after p2m_lookup() function, when random page of
>>>> guest domain memory is need to be mapped to xen. Data on this
>>>> already memory page may be not valid. Issue is fixed when
>>>> caches are invalidated after mapping is done.
>>>>
>>>> Signed-off-by: Andrii Tseglytskyi <andrii.tseglytskyi@xxxxxxxxxxxxxxx>
>>>> ---
>>>>    xen/arch/arm/mm.c |    2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
>>>> index 0a243b0..085780a 100644
>>>> --- a/xen/arch/arm/mm.c
>>>> +++ b/xen/arch/arm/mm.c
>>>> @@ -304,7 +304,7 @@ void *map_domain_page(unsigned long mfn)
>>>>         * We may not have flushed this specific subpage at map time,
>>>>         * since we only flush the 4k page not the superpage
>>>>         */
>>>> -    flush_xen_data_tlb_range_va_local(va, PAGE_SIZE);
>>>
>>>
>>>
>>> Why did you remove the flush TLB? It's requirement to make sure the VA
>>> will pointed to the right PA.
>>>
>>>> +    clean_and_invalidate_xen_dcache_va_range((void *)va, PAGE_SIZE);
>>>
>>>
>>>
>>> This is not the right approach, map_domain_page is heavily used to map
>>> hypercall data page. Those pages must reside in normal inner-cacheable
>>
>>
>> What if use map_domain_page() outside hypercall ?
>
>
> In general we require cache enabled for any communication with Xen. Until
> now, we didn't have any case where map_domain_page is used outside this
> scope.
>
>
>>> memory. So cleaning the cache is useless and time consuming.
>>
>>
>> I see that without cache invalidating page contains invalid data. Let
>> me explain more deeply:
>> As far as you know - I resumed my work with remoteproc MMU driver and
>> I started fixing review comments. One of your comments is that
>> ioremap_nocache() API should not be used for mapping domain pages,
>> therefore I tried using map_domain_page, and this works fine for me
>> only in case if I invalidate caches of already mapped va.
>>
>> I compared page dumps - 1 st page was mapped with ioremap_nocache()
>> function, second page was mapped with map_domain_page() function, and
>> I got the following output:
>>
>> (XEN) SGX_L2_MMU: pte_table[0] 0x9d428019 tmp[0] 0x9d428019
>> (XEN) SGX_L2_MMU: pte_table[1] 0x9d429019 tmp[1] 0x9d429019
>> (XEN) SGX_L2_MMU: pte_table[2] 0x9d42e019 tmp[2] 0x9d42e019
>> (XEN) SGX_L2_MMU: pte_table[3] 0x9d42f019 tmp[3] 0x00000000  <-- data
>> is not valid here
>>
>> pte_table pointer is mapped using ioremap_nocache(), tmp is mapped
>> using map_domain_page()
>>>
>>>
>>>
>>> If you want to clean and invalidate the cache, even though I don't think
>>> this right by reading the commit message, you have to introduce a new
>>> helper.
>>>
>>
>> Taking in account your previous comment - that map_domain_page() is
>> used for mapping of hypercall data page, looks like I can't use this
>> API as is. In my code I solved this by calling of
>> clean_and_invalidate_xen_dcache_va_range() immediately after page is
>> mapped. This works fine for me - no invalid data is observed.
>
>
> What is the attribute of this page in guest? non-cacheable?
>

It is domain heap memory. I expect it should be cacheable. How can I
make sure with this?


> I understand why you would need the invalidate, even though it's specific to
> your case. But not the clean...
> If the page is non-cacheable you may write stall data in the memory.
>
> Regards,
>
> --
> Julien Grall



-- 

Andrii Tseglytskyi | Embedded Dev
GlobalLogic
www.globallogic.com

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