Re: [PATCH v4 2/5] x86/asm: add enqcmds() to support ENQCMDS instruction

From: Dave Jiang
Date: Wed Sep 23 2020 - 11:47:35 EST

On 9/23/2020 4:08 AM, Borislav Petkov wrote:
On Thu, Sep 17, 2020 at 02:15:23PM -0700, Dave Jiang wrote:
Add enqcmds() in x86 io.h instead of special_insns.h.

Why? It is an asm wrapper for a special instruction.

Ok will move.

instruction can be used for other purposes. A wrapper was introduced
in io.h for its command submission usage. ENQCMDS has a single
purpose of submit 64-byte commands to supported devices and should
be called directly.

Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
arch/x86/include/asm/io.h | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)

diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index d726459d08e5..b7af0bf8a018 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -424,4 +424,33 @@ static inline void iosubmit_cmds512(void __iomem *dst, const void *src,
+ * enqcmds - copy a 512 bits data unit to single MMIO location

Your #319433 doc says

"ENQCMDS — Enqueue Command Supervisor"

Now *how* that enqueueing is done you can explain in the comment below.

Ok will add.

+ * @dst: destination, in MMIO space (must be 512-bit aligned)
+ * @src: source
+ *
+ * Submit data from kernel space to MMIO space, in a unit of 512 bits.
+ * Order of data access is not guaranteed, nor is a memory barrier
+ * performed afterwards. The command returns false (0) on failure, and true (1)
+ * on success.

The command or the function?

Function. Will fix.

From what I see below, the instruction sets ZF=1 to denote that it needs
to be retried and ZF=0 means success, as the doc says. And in good UNIX
tradition, 0 means usually success and !0 failure.

So why are you flipping that?

Ok will return 0 for success and -ERETRY for failure.

+ * Warning: Do not use this helper unless your driver has checked that the CPU
+ * instruction is supported on the platform.
+ */
+static inline bool enqcmds(void __iomem *dst, const void *src)
+ bool retry;
+ /* ENQCMDS [rdx], rax */
+ asm volatile(".byte 0xf3, 0x0f, 0x38, 0xf8, 0x02, 0x66, 0x90\t\n"
No need for those last two chars.

Ok will remove.

+ CC_SET(z)
+ : CC_OUT(z) (retry)
+ : "a" (dst), "d" (src));

<---- newline here.

Will fix.

+ /* Submission failure is indicated via EFLAGS.ZF=1 */
+ if (retry)
+ return false;
+ return true;
#endif /* _ASM_X86_IO_H */


Thank you very much for reviewing Boris. Very much appreciated!