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

[Borislav Petkov <bp@alien8.de>]

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?

> + * @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?

> + *
> + * 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*.

> +{
> +	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

  static_cpu_has(X86_FEATURE_MOVDIR64B)

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

-- 
Regards/Gruss,
    Boris.

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