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

From: Juergen Gross <jgross@suse.com>
To: jejb@linux.ibm.com,
	Sreekanth Reddy <sreekanth.reddy@broadcom.com>,
	Bart Van Assche <bvanassche@acm.org>
Cc: Salvatore Bonaccorso <carnil@debian.org>,
	sathya.prakash@broadcom.com,
	suganath-prabu.subramani@broadcom.com,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	MPT-FusionLinux.pdl@broadcom.com, linux-scsi@vger.kernel.org,
	linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
	adi@kriegisch.at
Subject: Re: Report in downstream Debian: mpt3sas broken with xen dom0 with update to 5.10.149 in 5.10.y.
Date: Mon, 24 Oct 2022 15:20:43 +0200	[thread overview]
Message-ID: <30a056c8-071f-4259-3253-75e718af619d@suse.com> (raw)
In-Reply-To: <2fd505a07bd26d76f1166761fa50905414edb7ef.camel@linux.ibm.com>

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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@acm.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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