Re: [bug] __blk_mq_run_hw_queue suspicious rcu usage

[Christoph Hellwig]

On Wed, Sep 04, 2019 at 02:40:44PM -0700, David Rientjes wrote:
> Hi Christoph, Jens, and Ming,
> 
> While booting a 5.2 SEV-enabled guest we have encountered the following 
> WARNING that is followed up by a BUG because we are in atomic context 
> while trying to call set_memory_decrypted:

Well, this really is a x86 / DMA API issue unfortunately.  Drivers
are allowed to do GFP_ATOMIC dma allocation under locks / rcu critical
sections and from interrupts.  And it seems like the SEV case can't
handle that.  We have some semi-generic code to have a fixed sized
pool in kernel/dma for non-coherent platforms that have similar issues
that we could try to wire up, but I wonder if there is a better way
to handle the issue, so I've added Tom and the x86 maintainers.

Now independent of that issue using DMA coherent memory for the nvme
PRPs/SGLs doesn't actually feel very optional.  We could do with
normal kmalloc allocations and just sync it to the device and back.
I wonder if we should create some general mempool-like helpers for that.