linux-kernel - Re: [PATCH v2 1/2] xen/swiotlb: add alignment check for dma buffers

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Message-ID: <alpine.DEB.2.22.394.2409161312000.1417852@ubuntu-linux-20-04-desktop>
Date: Mon, 16 Sep 2024 13:19:18 -0700 (PDT)
From: Stefano Stabellini <sstabellini@...nel.org>
To: Juergen Gross <jgross@...e.com>
cc: Jan Beulich <jbeulich@...e.com>, 
    Stefano Stabellini <sstabellini@...nel.org>, 
    Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>, 
    xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org, 
    iommu@...ts.linux.dev
Subject: Re: [PATCH v2 1/2] xen/swiotlb: add alignment check for dma
 buffers

On Mon, 16 Sep 2024, Juergen Gross wrote:
> On 16.09.24 08:56, Juergen Gross wrote:
> > On 16.09.24 08:50, Jan Beulich wrote:
> > > On 16.09.2024 08:47, Juergen Gross wrote:
> > > > --- a/drivers/xen/swiotlb-xen.c
> > > > +++ b/drivers/xen/swiotlb-xen.c
> > > > @@ -78,9 +78,15 @@ static inline int
> > > > range_straddles_page_boundary(phys_addr_t p, size_t size)
> > > >   {
> > > >       unsigned long next_bfn, xen_pfn = XEN_PFN_DOWN(p);
> > > >       unsigned int i, nr_pages = XEN_PFN_UP(xen_offset_in_page(p) +
> > > > size);
> > > > +    phys_addr_t algn = 1ULL << (get_order(size) + PAGE_SHIFT);
> > > >       next_bfn = pfn_to_bfn(xen_pfn);
> > > > +    /* If buffer is physically aligned, ensure DMA alignment. */
> > > > +    if (IS_ALIGNED(p, algn) &&
> > > > +        !IS_ALIGNED(next_bfn << XEN_PAGE_SHIFT, algn))
> > > 
> > > And this shift is not at risk of losing bits on Arm LPAE?
> > 
> > For alignment check this just doesn't matter (assuming XEN_PAGE_SIZE is
> > smaller than 4G).
> 
> Wait, that was nonsense.
> 
> I'll change the check to:
> 
> 	!IS_ALIGNED((phys_addr_t)next_bfn << XEN_PAGE_SHIFT, algn)

With this change:

Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>