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Date: Fri, 22 Jan 2021 09:08:06 -0800
From: Ben Widawsky <ben.widawsky@...el.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-pci@...r.kernel.org,
"linux-acpi@...r.kernel.org, Ira Weiny" <ira.weiny@...el.com>,
Dan Williams <dan.j.williams@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>,
"Kelley, Sean V" <sean.v.kelley@...el.com>,
Rafael Wysocki <rafael.j.wysocki@...el.com>,
Bjorn Helgaas <helgaas@...nel.org>,
Jon Masters <jcm@...masters.org>,
Chris Browy <cbrowy@...ry-design.com>,
Randy Dunlap <rdunlap@...radead.org>,
Christoph Hellwig <hch@...radead.org>,
daniel.lll@...baba-inc.com
Subject: Re: [RFC PATCH v3 10/16] cxl/mem: Add send command
On 21-01-22 11:43:57, Jonathan Cameron wrote:
> On Thu, 21 Jan 2021 10:15:46 -0800
> Ben Widawsky <ben.widawsky@...el.com> wrote:
>
> > On 21-01-14 17:10:38, Jonathan Cameron wrote:
> > > On Mon, 11 Jan 2021 14:51:14 -0800
> > > Ben Widawsky <ben.widawsky@...el.com> wrote:
> > >
> > > > The send command allows userspace to issue mailbox commands directly to
> > > > the hardware. The driver will verify basic properties of the command and
> > > > possible inspect the input (or output) payload to determine whether or
> > > > not the command is allowed (or might taint the kernel).
> > > >
> > > > The list of allowed commands and their properties can be determined by
> > > > using the QUERY IOCTL for CXL memory devices.
> > > >
> > > > Signed-off-by: Ben Widawsky <ben.widawsky@...el.com>
> > > > ---
> > > > drivers/cxl/mem.c | 204 ++++++++++++++++++++++++++++++++++-
> > > > include/uapi/linux/cxl_mem.h | 39 +++++++
> > > > 2 files changed, 239 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> > > > index d4eb3f5b9469..f979788b4d9f 100644
> > > > --- a/drivers/cxl/mem.c
> > > > +++ b/drivers/cxl/mem.c
> > > > @@ -84,6 +84,13 @@ static DEFINE_IDR(cxl_mem_idr);
> > > > /* protect cxl_mem_idr allocations */
> > > > static DEFINE_MUTEX(cxl_memdev_lock);
> > > >
> > > > +#undef C
> > > > +#define C(a, b) { b }
> > >
> > > I'm not following why this is here?
> > >
> >
> > It's used for a debug message in handle_mailbox_cmd_from_user(). This is all the
> > macro magic stolen from ftrace. Or, did I miss the question?
> >
> > > > +static struct {
> > > > + const char *name;
> > > > +} command_names[] = { CMDS };
> > > > +#undef C
>
> Mostly that you define it then undef it without use that I can see.
>
> > > > +
> > > > #define CXL_CMD(_id, _flags, sin, sout, f) \
> > > > [CXL_MEM_COMMAND_ID_##_id] = { \
> > > > { \
> > > ...
> > >
> > > > +
> > > > +/**
> > > > + * handle_mailbox_cmd_from_user() - Dispatch a mailbox command.
> > > > + * @cxlmd: The CXL memory device to communicate with.
> > > > + * @cmd: The validated command.
> > > > + * @in_payload: Pointer to userspace's input payload.
> > > > + * @out_payload: Pointer to userspace's output payload.
> > > > + * @u: The command submitted by userspace. Has output fields.
> > > > + *
> > > > + * Return:
> > > > + * * %0 - Mailbox transaction succeeded.
> > > > + * * %-EFAULT - Something happened with copy_to/from_user.
> > > > + * * %-EINTR - Mailbox acquisition interrupted.
> > > > + * * %-E2BIG - Output payload would overrun buffer.
> > > > + *
> > > > + * Creates the appropriate mailbox command on behalf of a userspace request.
> > > > + * Return value, size, and output payload are all copied out to @u. The
> > > > + * parameters for the command must be validated before calling this function.
> > > > + *
> > > > + * A 0 return code indicates the command executed successfully, not that it was
> > > > + * itself successful. IOW, the retval should always be checked if wanting to
> > >
> > > cmd->retval perhaps to be more explicit?
> > >
> > > > + * determine the actual result.
> > > > + */
> > > > +static int handle_mailbox_cmd_from_user(struct cxl_memdev *cxlmd,
> > > > + const struct cxl_mem_command *cmd,
> > > > + u64 in_payload,
> > > > + u64 out_payload,
> > > > + struct cxl_send_command __user *u)
> > > > +{
> > > > + struct mbox_cmd mbox_cmd = {
> > > > + .opcode = cmd->opcode,
> > > > + .size_in = cmd->info.size_in,
> > > > + .payload = NULL, /* Copied by copy_to|from_user() */
> > > > + };
> > > > + int rc;
> > > > +
> > > > + if (cmd->info.size_in) {
> > > > + /*
> > > > + * Directly copy the userspace payload into the hardware. UAPI
> > > > + * states that the buffer must already be little endian.
> > > > + */
> > > > + if (copy_from_user((__force void *)cxl_payload_regs(cxlmd->cxlm),
> > > > + u64_to_user_ptr(in_payload),
> > > > + cmd->info.size_in)) {
> > > > + cxl_mem_mbox_put(cxlmd->cxlm);
> > >
> > > mbox_get is after this point though it shouldn't be given we just
> > > wrote into the mbox registers.
> > >
> > > This seems unlikely to be a high performance path, so perhaps just
> > > use a local buffer and let cxl_mem_mbox_send_cmd copy it into the registers.
> > >
> >
> > You're correct about the get() needing to be first. I will fix it. As for
> > performance path - so while this does potentially help with performance, it
> > actually ends up being I think a little cleaner to not have to deal with a local
> > buffer.
> >
> > How strongly do you feel about it? I'd say if you don't care so much, let's keep
> > it as is and find a reason to undo later.
>
> A slightly interesting corner. The fact that there are no other cases of this
> particular sequence in kernel bothered me... It's more than possible I've
> missed something in the following.
>
> So with a bounce buffered we'd have
> copy_from_user()
> then
> memcpy_toio()
>
> here we end loosing the fact that memcpy_to_io() might not be a 'simple' memcpy().
> In the generic asm form it's just a (__force void *) like you have here done using
> __io_virt() (which might make sense here if you keep this, to make it clear
> what's going on)
>
> However, not all architectures are using the generic form of memcpy_toio()
> and even if the ones we care about are safe today using the above construct,
> it's more than possible some future architecture might be more 'exciting'.
>
> So basically I'm doubtful that this construct is safe.
>
> Jonathan
>
Sounds reasonable.
Thanks for digging. I'll go back to the bounce buffer in v4.
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