Re: [PATCH V6 08/10] cxl/cdat: Introduce cdat_hdr_valid()

[Ira Weiny]

On Tue, Feb 01, 2022 at 10:56:40AM -0800, Widawsky, Ben wrote:
> On 22-01-31 23:19:50, ira.weiny@xxxxxxxxx wrote:
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > The CDAT data is protected by a checksum which should be checked when
> > the CDAT is read to ensure it is valid.  In addition the lengths
> > specified should be checked.
> > 
> > Introduce cdat_hdr_valid() to check the checksum.  While at it check and
> > store the sequence number.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > ---
> > Changes from V5
> > 	New patch, split out
> > 	Update cdat_hdr_valid()
> > 		Remove revision and cs field parsing
> > 			There is no point in these
> > 		Add seq check and debug print.
> > ---
> >  drivers/cxl/cdat.h |  2 ++
> >  drivers/cxl/pci.c  | 32 ++++++++++++++++++++++++++++++++
> >  2 files changed, 34 insertions(+)
> > 
> > diff --git a/drivers/cxl/cdat.h b/drivers/cxl/cdat.h
> > index 4722b6bbbaf0..a7725d26f2d2 100644
> > --- a/drivers/cxl/cdat.h
> > +++ b/drivers/cxl/cdat.h
> > @@ -88,10 +88,12 @@
> >   *
> >   * @table: cache of CDAT table
> >   * @length: length of cached CDAT table
> > + * @seq: Last read Sequence number of the CDAT table
> >   */
> >  struct cxl_cdat {
> >  	void *table;
> >  	size_t length;
> > +	u32 seq;
> >  };
> >  
> >  #endif /* !__CXL_CDAT_H__ */
> > diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> > index 28b973a9e29e..c362c75feed2 100644
> > --- a/drivers/cxl/pci.c
> > +++ b/drivers/cxl/pci.c
> > @@ -586,6 +586,35 @@ static int cxl_setup_doe_devices(struct cxl_dev_state *cxlds)
> >  	return 0;
> >  }
> >  
> > +static bool cxl_cdat_hdr_valid(struct device *dev, struct cxl_cdat *cdat)
> > +{
> > +	u32 *table = cdat->table;
> > +	u8 *data8 = cdat->table;
> > +	u32 length, seq;
> > +	u8 check;
> > +	int i;
> > +
> > +	length = FIELD_GET(CDAT_HEADER_DW0_LENGTH, table[0]);
> > +	if (length < CDAT_HEADER_LENGTH_BYTES)
> > +		return false;
> > +
> > +	if (length > cdat->length)
> > +		return false;
> > +
> > +	seq = FIELD_GET(CDAT_HEADER_DW3_SEQUENCE, table[3]);
> > +
> > +	/* Store the sequence for now. */
> > +	if (cdat->seq != seq) {
> > +		dev_info(dev, "CDAT seq change %x -> %x\n", cdat->seq, seq);
> > +		cdat->seq = seq;
> > +	}
> 
> If sequence hasn't changed you could short-circuit the checksum.

I'm not sure.  Jonathan mentioned that reading may race with updates and that
the correct thing to do is re-read.[1]

But I should probably check the CS first...

Ira

[1] https://lore.kernel.org/linux-cxl/20211108145239.000010a5@xxxxxxxxxx/

> 
> > +
> > +	for (check = 0, i = 0; i < length; i++)
> > +		check += data8[i];
> > +
> > +	return check == 0;
> > +}
> > +
> >  #define CDAT_DOE_REQ(entry_handle)					\
> >  	(FIELD_PREP(CXL_DOE_TABLE_ACCESS_REQ_CODE,			\
> >  		    CXL_DOE_TABLE_ACCESS_REQ_CODE_READ) |		\
> > @@ -658,6 +687,9 @@ static int cxl_cdat_read_table(struct cxl_dev_state *cxlds,
> >  
> >  	} while (entry_handle != 0xFFFF);
> >  
> > +	if (!cxl_cdat_hdr_valid(cxlds->dev, cdat))
> > +		return -EIO;
> > +
> >  	return 0;
> >  }
> >  
> > -- 
> > 2.31.1
> >