linux-kernel - Re: [PATCH] x86/cpufeatures: Add enumeration for serialize instruction

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Message-Id: <5A5AF726-1D7C-4267-90D8-7E6BD464BA47@amacapital.net>
Date:   Mon, 6 Apr 2020 18:53:10 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ricardo Neri <ricardo.neri@...el.com>, X86 ML <x86@...nel.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH] x86/cpufeatures: Add enumeration for serialize instruction



> On Apr 6, 2020, at 6:37 PM, Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> wrote:
> 
> On Sat, Apr 04, 2020 at 02:15:57PM -0700, Andy Lutomirski wrote:
>>> On Fri, Apr 3, 2020 at 10:19 PM Ricardo Neri
>>> <ricardo.neri-calderon@...ux.intel.com> wrote:
>>> 
>>> On Fri, Apr 03, 2020 at 10:12:17AM +0200, Borislav Petkov wrote:
>>>> On Thu, Apr 02, 2020 at 06:40:26PM -0700, Ricardo Neri wrote:
>>>>> The serialize instruction ensures that before the next instruction is
>>>>> fetched and executed, all the modifications to flags, registers, and memory
>>>>> made by previous instructions are completed, draining all buffered writes
>>>>> to memory.
>>>>> 
>>>>> Importantly, the serialize instruction does not modify registers,
>>>>> arithmetic flags or memory.
>>>>> 
>>>>> Hence, the serialize instructions provides a better way for software
>>>>> to serialize execution than using instructions such as cpuid, which does
>>>>> modify registers and, in virtual machines, causes a VM exit.
>>>>> 
>>>>> This instruction is supported by the CPU if CPUID.7H.EDX[bit 14] is
>>>>> set.
>>>>> 
>>>>> Cc: x86@...nel.org
>>>>> Cc: "Ravi V. Shankar" <ravi.v.shankar@...el.com>
>>>>> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
>>>>> ---
>>>>> This new instruction is documented in the latest version of the Intel
>>>>> Architecture Instruction Set Extensions and Future Features Programming
>>>>> Reference Chapter 2.1 located at
>>>>> https://software.intel.com/sites/default/files/managed/c5/15/architecture-instruction-set-extensions-programming-reference.pdf
>>>>> ---
>>>>> arch/x86/include/asm/cpufeatures.h | 1 +
>>>>> 1 file changed, 1 insertion(+)
>>>>> 
>>>>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>>>>> index db189945e9b0..cd9b1ec022ec 100644
>>>>> --- a/arch/x86/include/asm/cpufeatures.h
>>>>> +++ b/arch/x86/include/asm/cpufeatures.h
>>>>> @@ -364,6 +364,7 @@
>>>>> #define X86_FEATURE_AVX512_VP2INTERSECT (18*32+ 8) /* AVX-512 Intersect for D/Q */
>>>>> #define X86_FEATURE_MD_CLEAR               (18*32+10) /* VERW clears CPU buffers */
>>>>> #define X86_FEATURE_TSX_FORCE_ABORT        (18*32+13) /* "" TSX_FORCE_ABORT */
>>>>> +#define X86_FEATURE_SERIALIZE              (18*32+14) /* SERIALIZE instruction */
>>>>> #define X86_FEATURE_PCONFIG                (18*32+18) /* Intel PCONFIG */
>>>>> #define X86_FEATURE_SPEC_CTRL              (18*32+26) /* "" Speculation Control (IBRS + IBPB) */
>>>>> #define X86_FEATURE_INTEL_STIBP            (18*32+27) /* "" Single Thread Indirect Branch Predictors */
>>>>> --
>>>> 
>>>> Send this together with code which is using it, pls.
>>> 
>>> Do you mean code in the kernel using this instructions. Thus far, I
>>> don't have any kernel use cases for this instruction. My intention is to expose
>>> this instruction to user space via /proc/cpuinfo. Is that not
>>> acceptable?
>> 
>> Presumably sync_core() should do, roughly:
>> 
>> if (static_cpu_has(X86_FEATURE_SERIALIZE)) {
>>  asm volatile("serialize");
>>  return;
>> }
> 
> Sure Andy, I will look at implementing something as you propose.
> 
>> 
>> but with the appropriate magic to build it on older binutils.  
> 
> But old binutils will not be aware of this new instruction, right? How
> could they be impacted?

Because old binutils will fail to assemble serialize :). You’re need a macro that turns it into .byte, or, if there’s just one user, you could open-code the .byte.

> 
>> should make sure that the in-kernel instruction decoder doesn't
>> explode when it sees serialize, presumably.
> 
> Sure Andy. I will also test for this.
> 
> Thanks and BR,
> Ricardo