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Date: Fri, 16 Feb 2024 13:10:14 +0100
From: Robert Richter <rrichter@....com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Alison Schofield <alison.schofield@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>,
Ira Weiny <ira.weiny@...el.com>,
Dan Williams <dan.j.williams@...el.com>,
Dave Jiang <dave.jiang@...el.com>,
Davidlohr Bueso <dave@...olabs.net>, linux-cxl@...r.kernel.org,
linux-kernel@...r.kernel.org,
"Rafael J. Wysocki" <rafael@...nel.org>,
Len Brown <lenb@...nel.org>, Lukas Wunner <lukas@...ner.de>,
Fan Ni <nifan.cxl@...il.com>
Subject: Re: [PATCH v3 2/3] cxl/pci: Get rid of pointer arithmetic reading
CDAT table
Hi Jonathan,
thanks for your review.
On 14.02.24 17:31:58, Jonathan Cameron wrote:
> On Fri, 9 Feb 2024 20:26:46 +0100
> Robert Richter <rrichter@....com> wrote:
>
> > Reading the CDAT table using DOE requires a Table Access Response
> > Header in addition to the CDAT entry. In current implementation this
> > has caused offsets with sizeof(__le32) to the actual buffers. This led
> > to hardly readable code and even bugs. E.g., see fix of devm_kfree()
> > in read_cdat_data():
> >
> > c65efe3685f5 cxl/cdat: Free correct buffer on checksum error
> >
> > Rework code to avoid calculations with sizeof(__le32). Introduce
> > struct cdat_doe_rsp for this which contains the Table Access Response
> > Header and a variable payload size for various data structures
> > afterwards to access the CDAT table and its CDAT Data Structures
> > without recalculating buffer offsets.
> >
> > Cc: Lukas Wunner <lukas@...ner.de>
> > Cc: Fan Ni <nifan.cxl@...il.com>
> > Reviewed-by: Dave Jiang <dave.jiang@...el.com>
> > Signed-off-by: Robert Richter <rrichter@....com>
>
> Hi Robert,
>
> I like this in general. A few comments inline though.
>
> > ---
> > drivers/cxl/core/pci.c | 75 ++++++++++++++++++++++--------------------
> > drivers/cxl/cxlpci.h | 20 +++++++++++
> > 2 files changed, 59 insertions(+), 36 deletions(-)
> >
> > diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> > index 39366ce94985..569354a5536f 100644
> > --- a/drivers/cxl/core/pci.c
> > +++ b/drivers/cxl/core/pci.c
> > @@ -544,55 +544,55 @@ static int cxl_cdat_get_length(struct device *dev,
> >
> > static int cxl_cdat_read_table(struct device *dev,
> > struct pci_doe_mb *doe_mb,
> > - void *cdat_table, size_t *cdat_length)
> > + struct cdat_doe_rsp *rsp, size_t *length)
>
> Nitpick, but rsp isn't a response, it's the whole table.
> Maybe it's worth a
> #define cdat_doe_table cdat_doe_rsp
> or a typedef so the two are different in name at least whilst sharing
> same structure definition?
There is a comment near the kzalloc of buf. I think introducing
another type here for single use will just add confusion.
I will also update the description of cdat_doe_rsp.
>
> > {
> > - size_t length = *cdat_length + sizeof(__le32);
> > - __le32 *data = cdat_table;
> > - int entry_handle = 0;
> > + size_t received, remaining = *length;
> > + unsigned int entry_handle = 0;
> > + union cdat_data *data;
> > __le32 saved_dw = 0;
> >
> > do {
> > __le32 request = CDAT_DOE_REQ(entry_handle);
> > - struct cdat_entry_header *entry;
> > - size_t entry_dw;
> > int rc;
> >
> > rc = pci_doe(doe_mb, PCI_DVSEC_VENDOR_ID_CXL,
> > CXL_DOE_PROTOCOL_TABLE_ACCESS,
> > &request, sizeof(request),
> > - data, length);
> > + rsp, sizeof(*rsp) + remaining);
>
> I guess it's not really worth using struct_size here.
> It's main advantage is making it clear we are dealing with a
> trailing []
Yes, will keep it as is. Since it's a u8 array, count is equal the
size for the remaining data and we do not need struct_size() here.
>
> > if (rc < 0) {
> > dev_err(dev, "DOE failed: %d", rc);
> > return rc;
> > }
> >
> > - /* 1 DW Table Access Response Header + CDAT entry */
> > - entry = (struct cdat_entry_header *)(data + 1);
> > - if ((entry_handle == 0 &&
> > - rc != sizeof(__le32) + sizeof(struct cdat_header)) ||
> > - (entry_handle > 0 &&
> > - (rc < sizeof(__le32) + sizeof(*entry) ||
> > - rc != sizeof(__le32) + le16_to_cpu(entry->length))))
> > + if (rc < sizeof(*rsp))
> > + return -EIO;
> > +
> > + data = (void *)rsp->data;
>
> Nicer to cast to (union cdat_data *) than rely on bounce via a void *
Will change.
>
> > + received = rc - sizeof(*rsp);
> > +
> > + if ((!entry_handle &&
>
> Prefer == 0 for this because 0 is a magic value here.
>
> > + received != sizeof(data->header)) ||
> > + (entry_handle &&
> > + (received < sizeof(data->entry) ||
> > + received != le16_to_cpu(data->entry.length))))
> > return -EIO;
>
> Given it's two rather involved conditions maybe better to do.
>
> if (entry_handle == 0) {
> if (received != sizeof(data->header)
> return -EIO;
> } else {
> if (received < sizeof(data->entry) ||
> received != le16_to_cpu(data->entry.length))
> return -EIO;
> }
>
> More code but easier to see the header vs entry checks.
> Could even define a little utility function / macro.
>
> cdat_is_head_handle(val) entry_handle == 0
> so you get somewhat more self documenting code.
>
> if (cdat_is_head_handle(entry_handle)) {
> } else {
> }
I will take this but without the macro.
>
> >
> > /* Get the CXL table access header entry handle */
> > entry_handle = FIELD_GET(CXL_DOE_TABLE_ACCESS_ENTRY_HANDLE,
> > - le32_to_cpu(data[0]));
> > - entry_dw = rc / sizeof(__le32);
> > - /* Skip Header */
> > - entry_dw -= 1;
> > + le32_to_cpu(rsp->doe_header));
> > +
> > /*
> > * Table Access Response Header overwrote the last DW of
> > * previous entry, so restore that DW
> > */
> > - *data = saved_dw;
> > - length -= entry_dw * sizeof(__le32);
> > - data += entry_dw;
> > - saved_dw = *data;
> > + rsp->doe_header = saved_dw;
>
> I'm not keen on this looking like we are writing the doe header
> as we are writing the tail of the last response.
>
> Maybe the comment is enough. I don't have a better idea on how
> to make this more obvious.
I think the comment is good enough here.
>
> > + remaining -= received;
> > + rsp = (void *)rsp + received;
>
> Was a potential problem with previous code, but this could
> in theory become unaligned and we should be using unaligned accessors
> for it as a result, or maybe adding a check that it doesn't ever become so.
> The check is probably the easier path given CDAT entries are thankfully
> (I think) all dword multiples as are the two headers.
Yes, buffers are dwords. In any case, pci_doe_recv_resp() is safe to
be used unaligned anyway.
Thanks for your review, will prepare a v4.
-Robert
>
> > + saved_dw = rsp->doe_header;
> > } while (entry_handle != CXL_DOE_TABLE_ACCESS_LAST_ENTRY);
> >
> > /* Length in CDAT header may exceed concatenation of CDAT entries */
> > - *cdat_length -= length - sizeof(__le32);
> > + *length -= remaining;
> >
> > return 0;
> > }
>
>
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