Re: [PATCH v6 2/5] x86/asm: Add enqcmds() to support ENQCMDS instruction

Dave Jiang
2020-09-24T21:15:15Z

On 9/24/2020 11:58 AM, Borislav Petkov wrote:
> On Thu, Sep 24, 2020 at 11:00:38AM -0700, Dave Jiang wrote:
>> +/**
>> + * enqcmds - copy a 512 bits data unit to single MMIO location
>
> You forgot to fix this.
>
>> + * @dst: destination, in MMIO space (must be 512-bit aligned)
>> + * @src: source
>> + *
>> + * The ENQCMDS instruction allows software to write a 512 bits command to
>> + * a 512 bits aligned special MMIO region that supports the instruction.
>> + * A return status is loaded into the ZF flag in the RFLAGS register.
>> + * ZF = 0 equates to success, and ZF = 1 indicates retry or error.
>> + *
>> + * The enqcmds() function uses the ENQCMDS instruction to 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
>> + * function returns 0 on success and -EAGAIN on failure.
>> + *
>> + * Warning: Do not use this helper unless your driver has checked that the CPU
>> + * instruction is supported on the platform and the device accepts ENQCMDS.
>> + */
>> +static inline int enqcmds(void __iomem *dst, const void *src)
>> +{
>> + int zf;
>> +
>> + /* ENQCMDS [rdx], rax */
>> + asm volatile(".byte 0xf3, 0x0f, 0x38, 0xf8, 0x02, 0x66, 0x90"
>> + CC_SET(z)
>> + : CC_OUT(z) (zf)
>> + : "a" (dst), "d" (src));
>
> Those operands need to be specified the same way as for movdir64b.
>
> I've done that to save roundtrip time - simply replace yours with this
> one after having tested it on actual hardware, of course.

Thanks Boris! Very much appreciate this. I have tested and everything is good.

>
> ---
> From 39cbdc81d657efcb73c0f7d7ab5e5c53f439f267 Mon Sep 17 00:00:00 2001
> From: Dave Jiang <dave.jiang@intel.com>
> Date: Thu, 24 Sep 2020 11:00:38 -0700
> Subject: [PATCH] x86/asm: Add an enqcmds() wrapper for the ENQCMDS instruction
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
>
> Currently, the MOVDIR64B instruction is used to atomically submit
> 64-byte work descriptors to devices. Although it can encounter errors
> like device queue full, command not accepted, device not ready, etc when
> writing to a device MMIO, MOVDIR64B can not report back on errors from
> the device itself. This means that MOVDIR64B users need to separately
> interact with a device to see if a descriptor was successfully queued,
> which slows down device interactions.
>
> ENQCMD and ENQCMDS also atomically submit 64-byte work descriptors
> to devices. But, they *can* report back errors directly from the
> device, such as if the device was busy, or device not enabled or does
> not support the command. This immediate feedback from the submission
> instruction itself reduces the number of interactions with the device
> and can greatly increase efficiency.
>
> ENQCMD can be used at any privilege level, but can effectively only
> submit work on behalf of the current process. ENQCMDS is a ring0-only
> instruction and can explicitly specify a process context instead of
> being tied to the current process or needing to reprogram the IA32_PASID
> MSR.
>
> Use ENQCMDS for work submission within the kernel because a Process
> Address ID (PASID) is setup to translate the kernel virtual address
> space. This PASID is provided to ENQCMDS from the descriptor structure
> submitted to the device and not retrieved from IA32_PASID MSR, which is
> setup for the current user address space.
>
> See Intel Software Developer’s Manual for more information on the
> instructions.
>
> [ bp:
> - Make operand constraints like movdir64b() because both insns are
> basically doing the same thing, more or less.
> - Fixup comments and cleanup. ]
>
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> Link: https://lkml.kernel.org/r/20200924180041.34056-3-dave.jiang@intel.com
> ---
> arch/x86/include/asm/special_insns.h | 42 ++++++++++++++++++++++++++++
> 1 file changed, 42 insertions(+)
>
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 2f0c8a39c796..2c18c780b2d5 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -262,6 +262,48 @@ static inline void movdir64b(void *dst, const void *src)
> : "m" (*__src), "a" (__dst), "d" (__src));
> }
>
> +/**
> + * enqcmds - Enqueue a command in supervisor (CPL0) mode
> + * @dst: destination, in MMIO space (must be 512-bit aligned)
> + * @src: 512 bits memory operand
> + *
> + * The ENQCMDS instruction allows software to write a 512-bit command to
> + * a 512-bit-aligned special MMIO region that supports the instruction.
> + * A return status is loaded into the ZF flag in the RFLAGS register.
> + * ZF = 0 equates to success, and ZF = 1 indicates retry or error.
> + *
> + * This function issues the ENQCMDS instruction to 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. It
> + * returns 0 on success and -EAGAIN on failure.
> + *
> + * Warning: Do not use this helper unless your driver has checked that the
> + * ENQCMDS instruction is supported on the platform and the device accepts
> + * ENQCMDS.
> + */
> +static inline int enqcmds(void __iomem *dst, const void *src)
> +{
> + const struct { char _[64]; } *__src = src;
> + struct { char _[64]; } *__dst = dst;
> + int zf;
> +
> + /*
> + * ENQCMDS %(rdx), rax
> + *
> + * See movdir64b()'s comment on operand specification.
> + */
> + asm volatile(".byte 0xf3, 0x0f, 0x38, 0xf8, 0x02, 0x66, 0x90"
> + CC_SET(z)
> + : CC_OUT(z) (zf), "+m" (*__dst)
> + : "m" (*__src), "a" (__dst), "d" (__src));
> +
> + /* Submission failure is indicated via EFLAGS.ZF=1 */
> + if (zf)
> + return -EAGAIN;
> +
> + return 0;
> +}
> +
> #endif /* __KERNEL__ */
>
> #endif /* _ASM_X86_SPECIAL_INSNS_H */
>