Re: [PATCH v1 2/3] x86/msr: Switch between WRMSRNS and WRMSR with the alternatives mechanism

["H. Peter Anvin" <hpa@zytor.com>]

On August 17, 2024 7:23:07 AM PDT, Borislav Petkov <bp@alien8.de> wrote:
>On August 16, 2024 8:52:51 PM GMT+03:00, Xin Li <xin@zytor.com> wrote:
>>On 8/9/2024 4:07 PM, Andrew Cooper wrote:
>>> On 07/08/2024 6:47 am, Xin Li (Intel) wrote:
>>>> From: Andrew Cooper <andrew.cooper3@citrix.com>
>>>> +/* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
>>>> +#define WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
>>>> +
>>>> +/* Non-serializing WRMSR, when available.  Falls back to a serializing WRMSR. */
>>>>   static __always_inline void wrmsrns(u32 msr, u64 val)
>>>>   {
>>>> -	__wrmsrns(msr, val, val >> 32);
>>>> +	/*
>>>> +	 * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant
>>>> +	 * DS prefix to avoid a trailing NOP.
>>>> +	 */
>>>> +	asm volatile("1: "
>>>> +		     ALTERNATIVE("ds wrmsr",
>>> 
>>> This isn't the version I presented, and there's no discussion of the
>>> alteration.
>>
>>I'm trying to implement wrmsr() as
>>
>>static __always_inline void wrmsr(u32 msr, u64 val)
>>{
>>	asm volatile("1: " ALTERNATIVE_2("wrmsr", WRMSRNS, X86_FEATURE_WRMSRNS,
>>					 "call asm_xen_write_msr", X86_FEATURE_XENPV)
>>		     "2: " _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
>>		     : : "c" (msr), "a" (val), "d" ((u32)(val >> 32)),
>>		     "D" (msr), "S" (val));
>>}
>>
>>
>>As the CALL instruction is 5-byte long, and we need to pad nop for both
>>WRMSR and WRMSRNS, what about not using segment prefix at all?
>
>The alternatives macros can handle arbitrary insn sizes - no need for any padding.
>
>

The padding avoids unnecessary NOPs.