RE: [PATCH] xen/swiotlb: correct the check for xen_destroy_contiguous_region

[Peng Fan <peng.fan@xxxxxxx>]

> Subject: Re: [PATCH] xen/swiotlb: correct the check for
> xen_destroy_contiguous_region
> 
> On 28.04.20 09:33, peng.fan@xxxxxxx wrote:
> > From: Peng Fan <peng.fan@xxxxxxx>
> >
> > When booting xen on i.MX8QM, met:
> > "
> > [    3.602128] Unable to handle kernel paging request at virtual address
> 0000000000272d40
> > [    3.610804] Mem abort info:
> > [    3.613905]   ESR = 0x96000004
> > [    3.617332]   EC = 0x25: DABT (current EL), IL = 32 bits
> > [    3.623211]   SET = 0, FnV = 0
> > [    3.626628]   EA = 0, S1PTW = 0
> > [    3.630128] Data abort info:
> > [    3.633362]   ISV = 0, ISS = 0x00000004
> > [    3.637630]   CM = 0, WnR = 0
> > [    3.640955] [0000000000272d40] user address but active_mm is
> swapper
> > [    3.647983] Internal error: Oops: 96000004 [#1] PREEMPT SMP
> > [    3.654137] Modules linked in:
> > [    3.677285] Hardware name: Freescale i.MX8QM MEK (DT)
> > [    3.677302] Workqueue: events deferred_probe_work_func
> > [    3.684253] imx6q-pcie 5f000000.pcie: PCI host bridge to bus 0000:00
> > [    3.688297] pstate: 60000005 (nZCv daif -PAN -UAO)
> > [    3.688310] pc : xen_swiotlb_free_coherent+0x180/0x1c0
> > [    3.693993] pci_bus 0000:00: root bus resource [bus 00-ff]
> > [    3.701002] lr : xen_swiotlb_free_coherent+0x44/0x1c0
> > "
> >
> > In xen_swiotlb_alloc_coherent, if !(dev_addr + size - 1 <= dma_mask)
> > or range_straddles_page_boundary(phys, size) are true, it will create
> > contiguous region. So when free, we need to free contiguous region use
> > upper check condition.
> 
> No, this will break PV guests on x86.

Could you share more details why alloc and free not matching for the check?

Thanks,
Peng.

> 
> I think there is something wrong with your setup in combination with the ARM
> xen_create_contiguous_region() implementation.
> 
> Stefano?
> 
> 
> Juergen
> 
> >
> > Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
> > ---
> >   drivers/xen/swiotlb-xen.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index b6d27762c6f8..ab96e468584f 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -346,8 +346,8 @@ xen_swiotlb_free_coherent(struct device *hwdev,
> size_t size, void *vaddr,
> >     /* Convert the size to actually allocated. */
> >     size = 1UL << (order + XEN_PAGE_SHIFT);
> >
> > -   if (!WARN_ON((dev_addr + size - 1 > dma_mask) ||
> > -                range_straddles_page_boundary(phys, size)) &&
> > +   if (((dev_addr + size - 1 > dma_mask) ||
> > +       range_straddles_page_boundary(phys, size)) &&
> >         TestClearPageXenRemapped(virt_to_page(vaddr)))
> >             xen_destroy_contiguous_region(phys, order);
> >
> >