Re: [PATCH 02/10] swiotlb-xen: remove start_dma_addr

[Julien Grall]

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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > ---
> > >    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