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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/08/2024 6:52 pm, Xin Li 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 prefix was a minor optimisation to avoid having a trailing nop at
the end.

When combined with a call, you need 3 prefixes on WRMSR and 2 prefixes
on WRMSRNS to make all options be 5 bytes long.

That said, there's already a paravirt hook for this, and if you're
looking to work around the code gen mess for that, then doing it like
this by doubling up into rdi and rsi isn't great either.

My suggestion, not that I've had time to experiment, was to change
paravirt to use a non-C ABI and have asm_xen_write_msr() recombine
edx:eax into rsi.  That way the top level wrmsr() retains sensible
codegen for native even when paravirt is active.

~Andrew