Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on movdir64b CPU instruction

From: Dave Jiang
Date: Wed Nov 20 2019 - 19:10:51 EST

On 11/20/19 2:53 PM, Borislav Petkov wrote:
On Wed, Nov 20, 2019 at 02:23:49PM -0700, Dave Jiang wrote:
+ * iosubmit_cmds512 - copy data to single MMIO location, in 512-bit units

Where is the alignment check on that data before doing the copying?

I'll add the check on the destination address. The call is modeled after __iowrite64_copy() / __iowrite32_copy() in lib/iomap_copy.c. Looks like those functions do not check for the alignment requirements either.

+ * @dst: destination, in MMIO space (must be 512-bit aligned)
+ * @src: source
+ * @count: number of 512 bits quantities to submit

Where's that check on the data?

I don't follow?

+ *
+ * Submit data from kernel space to MMIO space, in units of 512 bits at a
+ * time. Order of access is not guaranteed, nor is a memory barrier
+ * performed afterwards.
+ */
+static inline void iosubmit_cmds512(void __iomem *dst, const void *src,
+ size_t count)

An iosubmit function which returns void and doesn't tell its callers
whether it succeeded or not? That looks non-optimal to say the least.

Why isn't there a fallback function which to call when the CPU doesn't
support movdir64b?

Because then you can use alternative_call() and have the thing work
regardless of hardware support for MOVDIR*.

Looks like Tony answered this part.

+ const u8 *from = src;
+ const u8 *end = from + count * 64;
+ if (!cpu_has_write512())

If anything, that thing needs to go and you should use


as it looks to me like you would care about speed on this fast path?
Yes, no?

Yes thank you!