Re: [PATCH] xen/swiotlb: correct the check for xen_destroy_contiguous_region

[Joe Jin <joe.jin@oracle.com>]

On 4/28/20 10:25 AM, Konrad Rzeszutek Wilk wrote:
> On Tue, Apr 28, 2020 at 12:19:41PM +0200, Jürgen Groß wrote:
>> On 28.04.20 10:25, Peng Fan wrote:
> 
> Adding Joe Jin.
> 
> Joe, didn't you have some ideas on how this could be implemented?
> 
>>>> Subject: Re: [PATCH] xen/swiotlb: correct the check for
>>>> xen_destroy_contiguous_region
>>>>
>>>> On 28.04.20 09:33, peng.fan@nxp.com wrote:
>>>>> From: Peng Fan <peng.fan@nxp.com>
>>>>>
>>>>> When booting xen on i.MX8QM, met:
>>>>> "
>>>>> [    3.602128] Unable to handle kernel paging request at virtual address
>>>> 0000000000272d40
>>>>> [    3.610804] Mem abort info:
>>>>> [    3.613905]   ESR = 0x96000004
>>>>> [    3.617332]   EC = 0x25: DABT (current EL), IL = 32 bits
>>>>> [    3.623211]   SET = 0, FnV = 0
>>>>> [    3.626628]   EA = 0, S1PTW = 0
>>>>> [    3.630128] Data abort info:
>>>>> [    3.633362]   ISV = 0, ISS = 0x00000004
>>>>> [    3.637630]   CM = 0, WnR = 0
>>>>> [    3.640955] [0000000000272d40] user address but active_mm is
>>>> swapper
>>>>> [    3.647983] Internal error: Oops: 96000004 [#1] PREEMPT SMP
>>>>> [    3.654137] Modules linked in:
>>>>> [    3.677285] Hardware name: Freescale i.MX8QM MEK (DT)
>>>>> [    3.677302] Workqueue: events deferred_probe_work_func
>>>>> [    3.684253] imx6q-pcie 5f000000.pcie: PCI host bridge to bus 0000:00
>>>>> [    3.688297] pstate: 60000005 (nZCv daif -PAN -UAO)
>>>>> [    3.688310] pc : xen_swiotlb_free_coherent+0x180/0x1c0
>>>>> [    3.693993] pci_bus 0000:00: root bus resource [bus 00-ff]
>>>>> [    3.701002] lr : xen_swiotlb_free_coherent+0x44/0x1c0
>>>>> "
>>>>>
>>>>> In xen_swiotlb_alloc_coherent, if !(dev_addr + size - 1 <= dma_mask)
>>>>> or range_straddles_page_boundary(phys, size) are true, it will create
>>>>> contiguous region. So when free, we need to free contiguous region use
>>>>> upper check condition.
>>>>
>>>> No, this will break PV guests on x86.
>>>
>>> Could you share more details why alloc and free not matching for the check?
>>
>> xen_create_contiguous_region() is needed only in case:
>>
>> - the bus address is not within dma_mask, or
>> - the memory region is not physically contiguous (can happen only for
>>   PV guests)
>>
>> In any case it should arrange for the memory to be suitable for the
>> DMA operation, so to be contiguous and within dma_mask afterwards. So
>> xen_destroy_contiguous_region() should only ever called for areas
>> which match above criteria, as otherwise we can be sure
>> xen_create_contiguous_region() was not used for making the area DMA-able
>> in the beginning.

I agreed with Juergen's explanation, That is my understanding.

Peng, if panic caused by (dev_addr + size - 1 > dma_mask), you should check
how you get the addr, if memory created by xen_create_contiguous_region(),
memory must be with in [0 - dma_mask].

Thanks,
Joe

>>
>> And this is very important in the PV case, as in those guests the page
>> tables are containing the host-PFNs, not the guest-PFNS, and
>> xen_create_contiguous_region() will fiddle with host- vs. guest-PFN
>> arrangements, and xen_destroy_contiguous_region() is reverting this
>> fiddling. Any call of xen_destroy_contiguous_region() for an area it
>> was not intended to be called for might swap physical pages beneath
>> random virtual addresses, which was the reason for this test to be
>> added by me.
>>
>>
>> Juergen
>>
>>>
>>> Thanks,
>>> Peng.
>>>
>>>>
>>>> I think there is something wrong with your setup in combination with the ARM
>>>> xen_create_contiguous_region() implementation.
>>>>
>>>> Stefano?
>>>>
>>>>
>>>> Juergen
>>>>
>>>>>
>>>>> Signed-off-by: Peng Fan <peng.fan@nxp.com>
>>>>> ---
>>>>>    drivers/xen/swiotlb-xen.c | 4 ++--
>>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
>>>>> index b6d27762c6f8..ab96e468584f 100644
>>>>> --- a/drivers/xen/swiotlb-xen.c
>>>>> +++ b/drivers/xen/swiotlb-xen.c
>>>>> @@ -346,8 +346,8 @@ xen_swiotlb_free_coherent(struct device *hwdev,
>>>> size_t size, void *vaddr,
>>>>>    	/* Convert the size to actually allocated. */
>>>>>    	size = 1UL << (order + XEN_PAGE_SHIFT);
>>>>>
>>>>> -	if (!WARN_ON((dev_addr + size - 1 > dma_mask) ||
>>>>> -		     range_straddles_page_boundary(phys, size)) &&
>>>>> +	if (((dev_addr + size - 1 > dma_mask) ||
>>>>> +	    range_straddles_page_boundary(phys, size)) &&
>>>>>    	    TestClearPageXenRemapped(virt_to_page(vaddr)))
>>>>>    		xen_destroy_contiguous_region(phys, order);
>>>>>
>>>>>
>>>
>>