Re: [PATCH v4 2/5] x86/asm: add enqcmds() to support ENQCMDS instruction
From: Borislav Petkov
Date: Wed Sep 23 2020 - 07:08:48 EST
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.
> 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.
> + * @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?
>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?
> + * 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.
> + CC_SET(z)
> + : CC_OUT(z) (retry)
> + : "a" (dst), "d" (src));
<---- newline here.
> + /* Submission failure is indicated via EFLAGS.ZF=1 */
> + if (retry)
> + return false;
> + return true;
> #endif /* _ASM_X86_IO_H */