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

[Dexuan Cui]

> From: Petr Tesařík <petr@xxxxxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

-- Dexuan