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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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@xxxxxxxxxx>
> >>>
> >>> 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@xxxxxxxxxx>
> >> 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.

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