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

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Date:   Mon, 24 Oct 2022 15:20:43 +0200
From:   Juergen Gross <jgross@...e.com>
To:     jejb@...ux.ibm.com, Sreekanth Reddy <sreekanth.reddy@...adcom.com>,
        Bart Van Assche <bvanassche@....org>
Cc:     Salvatore Bonaccorso <carnil@...ian.org>,
        sathya.prakash@...adcom.com, suganath-prabu.subramani@...adcom.com,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        MPT-FusionLinux.pdl@...adcom.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        adi@...egisch.at
Subject: Re: Report in downstream Debian: mpt3sas broken with xen dom0 with
 update to 5.10.149 in 5.10.y.

On 24.10.22 14:51, James Bottomley wrote:
> On Mon, 2022-10-24 at 17:26 +0530, Sreekanth Reddy wrote:
>> On Sun, Oct 23, 2022 at 6:57 AM Bart Van Assche <bvanassche@....org>
>> 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@eldamar.lan/.
>>
>> This issue is getting observed after having the below patch changes,
>> 2b9aba0c5d58e141e32bb1bb4c7cd91d19f075b8 scsi: mpt3sas: Fix return
>> value check of dma_get_required_mask()
>>
>> 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.
> 
> This sounds entirely correct because the VM is booted with (from the
> original debian bug report):
> 
> dom0_mem=4096M,max:4096M dom0_max_vcpus=4 dom0_vcpus_pin
>    ucode=scan xpti=dom0=false,domu=true gnttab_max_frames=128
> 
> So it has no memory above 4GB and thus 32 bit addressing is the minimum
> required.  If you boot a machine with >4GB and Xen still returns a 32
> bit mask here, then we have a Xen problem.

Dom0 is (normally) a PV domain, so the physical memory can be still above
4 GB even with dom0_mem set to 4GB.

But please see my other reply to this thread. I think at least one of the
backports is plain wrong.


Juergen

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