Re: [RFC PATCH v8 07/14] arm64/mm, xpfo: temporarily map dcache regions

[Khalid Aziz <khalid.aziz@oracle.com>]

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On 2/14/19 8:54 AM, Tycho Andersen wrote:
> Hi,
> 
> On Wed, Feb 13, 2019 at 05:01:30PM -0700, Khalid Aziz wrote:
>> From: Juerg Haefliger <juerg.haefliger@canonical.com>
>>
>> If the page is unmapped by XPFO, a data cache flush results in a fatal
>> page fault, so let's temporarily map the region, flush the cache, and then
>> unmap it.
>>
>> v6: actually flush in the face of xpfo, and temporarily map the underlying
>>     memory so it can be flushed correctly
>>
>> CC: linux-arm-kernel@lists.infradead.org
>> Signed-off-by: Juerg Haefliger <juerg.haefliger@canonical.com>
>> Signed-off-by: Tycho Andersen <tycho@docker.com>
>> ---
>>  arch/arm64/mm/flush.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
>> index 30695a868107..fad09aafd9d5 100644
>> --- a/arch/arm64/mm/flush.c
>> +++ b/arch/arm64/mm/flush.c
>> @@ -20,6 +20,7 @@
>>  #include <linux/export.h>
>>  #include <linux/mm.h>
>>  #include <linux/pagemap.h>
>> +#include <linux/xpfo.h>
>>  
>>  #include <asm/cacheflush.h>
>>  #include <asm/cache.h>
>> @@ -28,9 +29,15 @@
>>  void sync_icache_aliases(void *kaddr, unsigned long len)
>>  {
>>  	unsigned long addr = (unsigned long)kaddr;
>> +	unsigned long num_pages = XPFO_NUM_PAGES(addr, len);
>> +	void *mapping[num_pages];
> 
> What version does this build on? Presumably -Wvla will cause an error
> here, but,
> 
>>  	if (icache_is_aliasing()) {
>> +		xpfo_temp_map(kaddr, len, mapping,
>> +			      sizeof(mapping[0]) * num_pages);
>>  		__clean_dcache_area_pou(kaddr, len);
> 
> Here, we map the pages to some random address via xpfo_temp_map(),
> then pass the *original* address (which may not have been mapped) to
> __clean_dcache_area_pou(). So I think this whole approach is wrong.
> 
> If we want to do it this way, it may be that we need some
> xpfo_map_contiguous() type thing, but since we're just going to flush
> it anyway, that seems a little crazy. Maybe someone who knows more
> about arm64 knows a better way?
> 
> Tycho
> 

Hi Tycho,

You are right. Things don't quite look right with this patch. I don't
know arm64 well enough either, so I will wait for someone more
knowledgeable to make a recommendation here.

On a side note, do you mind if I update your address in your
signed-off-by from tycho@docker.com when I send the next version of this
series?

Thanks,
Khalid

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