linux-kernel - RE: [PATCH v1 2/2] swiotlb: Fix slot alignment checks

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Date:   Wed, 5 Apr 2023 05:11:42 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Petr Tesařík <petr@...arici.cz>
CC:     Dexuan-Linux Cui <dexuan.linux@...il.com>,
        Petr Tesarik <petrtesarik@...weicloud.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jianxiong Gao <jxgao@...gle.com>,
        David Stevens <stevensd@...omium.org>,
        Joerg Roedel <jroedel@...e.de>,
        "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
        open list <linux-kernel@...r.kernel.org>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH v1 2/2] swiotlb: Fix slot alignment checks

> From: Petr Tesařík <petr@...arici.cz>
> Sent: Tuesday, April 4, 2023 9:40 PM
> > > ...
> > > Hi Petr, this patch has gone into the mainline:
> > > 0eee5ae10256 ("swiotlb: fix slot alignment checks")
> > >
> > > Somehow it breaks Linux VMs on Hyper-V: a regular VM with
> > > swiotlb=force or a confidential VM (which uses swiotlb) fails to boot.
> > > If I revert this patch, everything works fine.
> >
> > The log is pasted below. Looks like the SCSI driver hv_storvsc fails to
> > detect the disk capacity:
> 
> The first thing I can imagine is that there are in fact no (free) slots
> in the SWIOTLB which match the alignment constraints, so the map
> operation fails. However, this would result in a "swiotlb buffer is
> full" message in the log, and I can see no such message in the log
> excerpt you have posted.
> 
> Please, can you check if there are any "swiotlb" messages preceding the
> first error message?
> 
> Petr T

There is no "swiotlb buffer is full" error.

The hv_storvsc driver (drivers/scsi/storvsc_drv.c) calls scsi_dma_map(),
which doesn't return -ENOMEM when the failure happens.

BTW, Kelsey reported the same issue (also no "swiotlb buffer is full" error):
https://lwn.net/ml/linux-kernel/20230405003549.GA21326@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net/

-- Dexuan