Re: [PATCH v2 3/3] block: sed-opal: Cache-line-align the cmd/resp buffers

[Serge Semin]

On Mon, Oct 03, 2022 at 12:24:08PM -0600, Jonathan Derrick wrote:
> Hi
> 
> On 9/29/2022 4:46 PM, Serge Semin wrote:
> > In accordance with [1] the DMA-able memory buffers must be
> > cacheline-aligned otherwise the cache writing-back and invalidation
> > performed during the mapping may cause the adjacent data being lost. It's
> > specifically required for the DMA-noncoherent platforms. Seeing the
> > opal_dev.{cmd,resp} buffers are used for DMAs in the NVME and SCSI/SD
> > drivers in framework of the nvme_sec_submit() and sd_sec_submit() methods
> > respectively we must make sure the passed buffers are cacheline-aligned to
> > prevent the denoted problem.
> > 
> > [1] Documentation/core-api/dma-api.rst
> > 
> > Fixes: 455a7b238cd6 ("block: Add Sed-opal library")
> > Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
> > ---
> >   block/sed-opal.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/sed-opal.c b/block/sed-opal.c
> > index 9700197000f2..222acbd1f03a 100644
> > --- a/block/sed-opal.c
> > +++ b/block/sed-opal.c
> > @@ -73,6 +73,7 @@ struct parsed_resp {
> >   	struct opal_resp_tok toks[MAX_TOKS];
> >   };
> > +/* Presumably DMA-able buffers must be cache-aligned */
> >   struct opal_dev {
> >   	bool supported;
> >   	bool mbr_enabled;
> > @@ -88,8 +89,8 @@ struct opal_dev {
> >   	u64 lowest_lba;
> >   	size_t pos;
> > -	u8 cmd[IO_BUFFER_LENGTH];
> > -	u8 resp[IO_BUFFER_LENGTH];
> > +	u8 cmd[IO_BUFFER_LENGTH] ____cacheline_aligned;
> > +	u8 resp[IO_BUFFER_LENGTH] ____cacheline_aligned;

> I'm with Christoph on this one.
> When I see ____cacheline_aligned, I assume its for performance reasons, not
> to work around a DMA limitation. Can we instead kmalloc (which provides
> alignment) these buffers to make it more clear? May want to add that same
> comment pointing out some architectures require these dma targets to be
> cache aligned.

Ok. I'll resend v3 with these buffers being kmalloc'ed.

Please note the SED OPAL entry of the MAINTAINTER list contains your
intel-email address, which bounces back the messages (so does the
Revanth' one). I'll add your new address to my patchset' "To"-list,
but if you want to get new OPAL-related patches sent directly to your
linux.dev email address the entry should be updated.

-Sergey

> 
> 
> >   	struct parsed_resp parsed;
> >   	size_t prev_d_len;