netdev - min_align_mask Re: [PATCH V3 13/13] HV/Storvsc: Add Isolation VM support for storvsc driver

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Message-ID: <20210824084934.GC29844@lst.de>
Date:   Tue, 24 Aug 2021 10:49:34 +0200
From:   "hch@....de" <hch@....de>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     "hch@....de" <hch@....de>, Tianyu Lan <ltykernel@...il.com>,
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Subject: min_align_mask Re: [PATCH V3 13/13] HV/Storvsc: Add Isolation VM
 support for storvsc driver

On Fri, Aug 20, 2021 at 03:40:08PM +0000, Michael Kelley wrote:
> I see that the swiotlb code gets and uses the min_align_mask field.  But
> the NVME driver is the only driver that ever sets it, so the value is zero
> in all other cases.  Does swiotlb just use PAGE_SIZE in that that case?  I
> couldn't tell from a quick glance at the swiotlb code.

The encoding isn't all that common.  I only know it for the RDMA memory
registration format, and RDMA in general does not interact very well
with swiotlb (although the in-kernel drivers should work fine, it is
userspace RDMA that is the problem).  It seems recently a new driver
using the format (mpi3mr) also showed up.  All these should probably set
the min_align_mask.