Re: Report in downstream Debian: mpt3sas broken with xen dom0 with update to 5.10.149 in 5.10.y.

[Adi Kriegisch]

Hi,


On Mon, Oct 24, 2022 at 05:26:44PM +0530, Sreekanth Reddy wrote:
> On Sun, Oct 23, 2022 at 6:57 AM Bart Van Assche <bvanassche@xxxxxxx> wrote:
> >
> > On 10/21/22 02:22, Salvatore Bonaccorso wrote:
> > > We got the following report in Debian after an update from 5.10.140 to
> > > the current 5.10.149. Full quoting below (from
> > > https://bugs.debian.org/1022126). Does this ring some bell about known
> > > regressions?
> >
> > Only three mpt3sas changes are new in v5.10.149 compared to v5.10.140:
> > $ git log --format=oneline v5.10.140..v5.10.149
> > 2b9aba0c5d58e141e32bb1bb4c7cd91d19f075b8 scsi: mpt3sas: Fix return value check of dma_get_required_mask()
> > e7fafef9830c4a01e60f76e3860a9bef0262378d scsi: mpt3sas: Force PCIe scatterlist allocations to be within same 4 GB region
> > ea10a652ad2ae2cf3eced6f632a5c98f26727057 scsi: mpt3sas: Fix use-after-free warning
> >
> > Sreekanth and Suganath, can you help with bisecting this issue? For the
> > full report, see also https://lore.kernel.org/linux-scsi/Y1JkuKTjVYrOWbvm@xxxxxxxxxxx/.
> 
> This issue is getting observed after having the below patch changes,
> 2b9aba0c5d58e141e32bb1bb4c7cd91d19f075b8 scsi: mpt3sas: Fix return
> value check of dma_get_required_mask()
reverting this patch fixed the issue for us.
 
> What is happening is that on Xen hypervisor, this
> dma_get_required_mask() API always returns a 32 bit DMA mask. I.e. It
> says that the minimum DMA mask required to access the host memory is
> 32 bit and hence mpt3sas driver is setting the DMA mask to 32bit. So,
> on a 64 bit machine, if the driver set's the DMA mask to 32 bit then
> SWIOTLB's bounce buffer comes into picture during IOs. Since these
> bounce buffers are limited in size and hence we observe the IO hang if
> the large IOs are issued.
I am not sure about what is going on here but while reverting the above
patch, I added a printk right above the if:
  | printk("dma_get_required_mask =%lld\n",dma_get_required_mask(&pdev->dev));
which always (as in booting dom0 with 2GB, 4GB, 16GB and booting bare metal
with 256GB) returns 4294967295 (DMA_BIT_MASK(32)).

> I am not sure whether this API's return value is correct or not in the
> Xen environment. If it is correct then I have to modify the driver to
> not use this API and directly set the DMA mask to 64 bit if the system
> is a 64bit machine.
Obviously, our server always reports a 32bit mask which works just fine in
the bare metal case but does not in the xen dom0 case. Is there anything I
can do to help tracking the issue down?

thanks and all the best,
    Adi
 
> Thanks,
> Sreekanth
> 
> 
> 
> >
> > Thanks,
> >
> > Bart.

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