Re: [PATCH 6/7] xen-gntdev: Support mapping in HVM domains

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

On 01/10/2011 05:41 PM, Konrad Rzeszutek Wilk wrote:
>> @@ -284,8 +304,25 @@ static void unmap_grant_pages(struct grant_map *map, int offset, int pages)
>>  		goto out;
>>  
>>  	for (i = 0; i < pages; i++) {
>> +		uint32_t check, *tmp;
>>  		WARN_ON(unmap_ops[i].status);
>> -		__free_page(map->pages[offset+i]);
>> +		if (!map->pages[i])
>> +			continue;
>> +		/* XXX When unmapping, Xen will sometimes end up mapping the GFN
>> +		 * to an invalid MFN. In this case, writes will be discarded and
>> +		 * reads will return all 0xFF bytes. Leak these unusable GFNs
>> +		 * until a way to restore them is found.
>> +		 */
>> +		tmp = kmap(map->pages[i]);
>> +		tmp[0] = 0xdeaddead;
>> +		mb();
>> +		check = tmp[0];
>> +		kunmap(map->pages[i]);
>> +		if (check == 0xdeaddead)
>> +			__free_page(map->pages[i]);
>> +		else if (debug)
>> +			printk("%s: Discard page %d=%ld\n", __func__,
>> +				i, page_to_pfn(map->pages[i]));
> 
> Whoa. Any leads to when the "sometimes" happens? Does the status report an
> error or is it silent?

Status is silent in this case. I can produce it quite reliably on my
test system where I am mapping a framebuffer (1280 pages) between two
HVM guests - in this case, about 2/3 of the released pages will end up
being invalid. It doesn't seem to be size-related as I have also seen
it on the small 3-page page index mapping. There is a message on xm
dmesg that may be related:

(XEN) sh error: sh_remove_all_mappings(): can't find all mappings of mfn 7cbc6: c=8000000000000004 t=7400000000000002

This appears about once per page, with different MFNs but the same c/t.
One of the two HVM guests (the one doing the mapping) has the PCI
graphics card forwarded to it.

>>  		map->pages[offset+i] = NULL;
>>  		map->pginfo[offset+i].handle = 0;
>>  	}
>