linux-kernel - Re: [PATCH v10 03/38] x86/msr: Add the WRMSRNS instruction support

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Date:   Thu, 14 Sep 2023 15:05:41 +0100
From:   andrew.cooper3@...rix.com
To:     Xin Li <xin3.li@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-hyperv@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        peterz@...radead.org, jgross@...e.com, ravi.v.shankar@...el.com,
        mhiramat@...nel.org, jiangshanlai@...il.com
Subject: Re: [PATCH v10 03/38] x86/msr: Add the WRMSRNS instruction support

On 14/09/2023 5:47 am, Xin Li wrote:
> Add an always inline API __wrmsrns() to embed the WRMSRNS instruction
> into the code.
>
> Tested-by: Shan Kang <shan.kang@...el.com>
> Signed-off-by: Xin Li <xin3.li@...el.com>
> ---
>  arch/x86/include/asm/msr.h | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 65ec1965cd28..c284ff9ebe67 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -97,6 +97,19 @@ static __always_inline void __wrmsr(unsigned int msr, u32 low, u32 high)
>  		     : : "c" (msr), "a"(low), "d" (high) : "memory");
>  }
>  
> +/*
> + * WRMSRNS behaves exactly like WRMSR with the only difference being
> + * that it is not a serializing instruction by default.
> + */
> +static __always_inline void __wrmsrns(u32 msr, u32 low, u32 high)
> +{
> +	/* Instruction opcode for WRMSRNS; supported in binutils >= 2.40. */
> +	asm volatile("1: .byte 0x0f,0x01,0xc6\n"
> +		     "2:\n"
> +		     _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
> +		     : : "c" (msr), "a"(low), "d" (high));
> +}
> +
>  #define native_rdmsr(msr, val1, val2)			\
>  do {							\
>  	u64 __val = __rdmsr((msr));			\
> @@ -297,6 +310,11 @@ do {							\
>  
>  #endif	/* !CONFIG_PARAVIRT_XXL */
>  
> +static __always_inline void wrmsrns(u32 msr, u64 val)
> +{
> +	__wrmsrns(msr, val, val >> 32);
> +}

This API works in terms of this series where every WRMSRNS is hidden
behind a FRED check, but it's an awkward interface to use anywhere else
in the kernel.

I fully understand that you expect all FRED capable systems to have
WRMSRNS, but it is not a hard requirement and you will end up with
simpler (and therefore better) logic by deleting the dependency.

As a "normal" user of the WRMSR APIs, the programmer only cares about:

1) wrmsr() -> needs to be serialising
2) wrmsr_ns() -> safe to be non-serialising


In Xen, I added something of the form:

/* Non-serialising WRMSR, when available.  Falls back to a serialising
WRMSR. */
static inline void wrmsr_ns(uint32_t msr, uint32_t lo, uint32_t hi)
{
    /*
     * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant CS
     * prefix to avoid a trailing NOP.
     */
    alternative_input(".byte 0x2e; wrmsr",
                      ".byte 0x0f,0x01,0xc6", X86_FEATURE_WRMSRNS,
                      "c" (msr), "a" (lo), "d" (hi));
}

and despite what Juergen said, I'm going to recommend that you do wire
this through the paravirt infrastructure, for the benefit of regular
users having a nice API, not because XenPV is expecting to do something
wildly different here.


I'd actually go as far as suggesting that you break patches 1-3 into
different series and e.g. update the regular context switch path to use
the WRMSRNS-falling-back-to-WRMSR helpers, just to get started.

~Andrew