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

[Dave Jiang]

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.

> > MOVDIR64B
> > 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 */
>
> Thx.

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