linux-kernel - Re: [PATCH 02/10] swiotlb-xen: remove start_dma

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Date:   Fri, 22 May 2020 19:16:06 +0100
From:   Julien Grall <julien@....org>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     jgross@...e.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>
Subject: Re: [PATCH 02/10] swiotlb-xen: remove start_dma_addr

Hi,

On 22/05/2020 04:55, Stefano Stabellini wrote:
> On Thu, 21 May 2020, Julien Grall wrote:
>> Hi,
>>
>> On 21/05/2020 00:45, Stefano Stabellini wrote:
>>> From: Stefano Stabellini <stefano.stabellini@...inx.com>
>>>
>>> It is not strictly needed. Call virt_to_phys on xen_io_tlb_start
>>> instead. It will be useful not to have a start_dma_addr around with the
>>> next patches.
>>>
>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
>>> ---
>>>    drivers/xen/swiotlb-xen.c | 5 +----
>>>    1 file changed, 1 insertion(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
>>> index a42129cba36e..b5e0492b07b9 100644
>>> --- a/drivers/xen/swiotlb-xen.c
>>> +++ b/drivers/xen/swiotlb-xen.c
>>> @@ -52,8 +52,6 @@ static unsigned long xen_io_tlb_nslabs;
>>>     * Quick lookup value of the bus address of the IOTLB.
>>>     */
>>>    -static u64 start_dma_addr;
>>> -
>>>    /*
>>>     * Both of these functions should avoid XEN_PFN_PHYS because phys_addr_t
>>>     * can be 32bit when dma_addr_t is 64bit leading to a loss in
>>> @@ -241,7 +239,6 @@ int __ref xen_swiotlb_init(int verbose, bool early)
>>>    		m_ret = XEN_SWIOTLB_EFIXUP;
>>>    		goto error;
>>>    	}
>>> -	start_dma_addr = xen_virt_to_bus(xen_io_tlb_start);
>>>    	if (early) {
>>>    		if (swiotlb_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs,
>>>    			 verbose))
>>> @@ -389,7 +386,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device
>>> *dev, struct page *page,
>>>    	 */
>>>    	trace_swiotlb_bounced(dev, dev_addr, size, swiotlb_force);
>>>    -	map = swiotlb_tbl_map_single(dev, start_dma_addr, phys,
>>> +	map = swiotlb_tbl_map_single(dev, virt_to_phys(xen_io_tlb_start),
>>> phys,
>>
>> xen_virt_to_bus() is implemented as xen_phys_to_bus(virt_to_phys()). Can you
>> explain how the two are equivalent?
> 
> They are not equivalent. Looking at what swiotlb_tbl_map_single expects,
> and also the implementation of swiotlb_init_with_tbl, I think
> virt_to_phys is actually the one we want.
> 
> swiotlb_tbl_map_single compares the argument with __pa(tlb) which is
> __pa(xen_io_tlb_start) which is virt_to_phys(xen_io_tlb_start).

I can't find such check in master. What is your baseline? Could you 
point to the exact line of code?

Cheers,

-- 
Julien Grall