2023-09-14 07:16:37

by Juergen Gross

[permalink] [raw]

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

---

On 14.09.23 06:47, Xin Li wrote:
> Add an always inline API __wrmsrns() to embed the WRMSRNS instruction
> into the code.
>
> Tested-by: Shan Kang <[email protected]>
> Signed-off-by: Xin Li <[email protected]>

In order to avoid having to add paravirt support for WRMSRNS I think
xen_init_capabilities() should gain:

+ setup_clear_cpu_cap(X86_FEATURE_WRMSRNS);


Juergen

> ---
> 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);
> +}
> +
> /*
> * 64-bit version of wrmsr_safe():
> */

---

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2023-09-14 15:10:57

by Andrew Cooper

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

---

On 14/09/2023 7:02 am, Juergen Gross wrote:
> On 14.09.23 06:47, Xin Li wrote:
>> Add an always inline API __wrmsrns() to embed the WRMSRNS instruction
>> into the code.
>>
>> Tested-by: Shan Kang <[email protected]>
>> Signed-off-by: Xin Li <[email protected]>
>
> In order to avoid having to add paravirt support for WRMSRNS I think
> xen_init_capabilities() should gain:
>
> +    setup_clear_cpu_cap(X86_FEATURE_WRMSRNS);

Xen PV guests will never ever see WRMSRNS.  Operating in CPL3, they have
no possible way of adjusting an MSR which isn't serialising, because
even the hypercall forms are serialising.

Xen only exposes the bit for HVM guests.

~Andrew