Re: [Xen-devel] [PATCH v2] xen/swiotlb: don't initialize swiotlb twice on arm64

[Juergen Gross <jgross@suse.com>]

On 13.06.19 16:23, Konrad Rzeszutek Wilk wrote:
> On Wed, Jun 05, 2019 at 04:24:06PM +0200, Juergen Gross wrote:
>> On 05.06.19 16:13, Konrad Rzeszutek Wilk wrote:
>>> On Tue, Jun 04, 2019 at 03:41:40PM -0400, Boris Ostrovsky wrote:
>>>> On 6/4/19 12:51 PM, Stefano Stabellini wrote:
>>>>> On Mon, 3 Jun 2019, Boris Ostrovsky wrote:
>>>>>> On 6/3/19 2:25 PM, Stefano Stabellini wrote:
>>>>>>> On Tue, 28 May 2019, Boris Ostrovsky wrote:
>>>>>>>> On 5/28/19 6:48 PM, Stefano Stabellini wrote:
>>>>>>>>> From: Stefano Stabellini <stefanos@xilinx.com>
>>>>>>>>>
>>>>>>>>> On arm64 swiotlb is often (not always) already initialized by mem_init.
>>>>>>>>> We don't want to initialize it twice, which would trigger a second
>>>>>>>>> memory allocation. Moreover, the second memory pool is typically made of
>>>>>>>>> high pages and ends up replacing the original memory pool of low pages.
>>>>>>>>> As a side effect of this change, it is possible to have low pages in
>>>>>>>>> swiotlb-xen on arm64.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
>>>>>>>> Has this been tested on x86?
>>>>>>> Yes, I managed to test it using QEMU. There are no effects on x86, as
>>>>>>> the check io_tlb_start != 0 returns false.
>>>>>> I wonder though whether this is always the case.  When we are called
>>>>>> from pci_xen_swiotlb_init_late() for example.
>>>>> In that case, pci_xen_swiotlb_init_late() is called by
>>>>> pcifront_connect_and_init_dma, which does:
>>>>>
>>>>> 	if (!err && !swiotlb_nr_tbl()) {
>>>>> 		err = pci_xen_swiotlb_init_late();
>>>>> 		if (err)
>>>>> 			dev_err(&pdev->xdev->dev, "Could not setup SWIOTLB!\n");
>>>>> 	}
>>>>>
>>>>> pci_xen_swiotlb_init_late() is only called when swiotlb_nr_tbl() returns
>>>>> 0. If swiotlb_nr_tbl() returns 0, certainly the swiotlb has not been
>>>>> allocated yet, and the io_tlb_start != 0 check at the beginning of
>>>>> xen_swiotlb_init will also fail. The code will take the normal
>>>>> route, same as today. In short, there should be no effects on x86.
>>>>
>>>>
>>>> OK, thanks.
>>>>
>>>> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>>
>>> Pushed in devel/for-linus-5.2 and will eventually move it to stable and push to Linus next-week.
>>>
>>> Are there any other patches I should pick up?
>>>
>>
>> I think at least the first two patches from my series:
>>
>> https://patchew.org/Xen/20190529090407.1225-1-jgross@suse.com/
>>
>> are ready to go in.
> 
> #2 patch says:
> 
> 	"> To be symmetric with setting the flag only after having made the region
> 	> contiguous, and to avoid (perhaps just theoretical) races, wouldn't it be
> 	> better to clear the flag before calling xen_destroy_contiguous_region()?
> 	> Even better would be a TestAndClear...() operation.
> 
> 	I like that idea.
> "
> ?

I was hoping for a clarification regarding the Xen specific page flag
names before posting V3.

As Christoph didn't react when I posted possible solutions I think I'll
just modify patch 3 according to Jan's comment and post V3.


Juergen

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