Re: [PATCH v1 2/4] x86/fred: Write to FRED MSRs with wrmsrns()

H. Peter Anvin
2024-07-03T16:07:44Z

On July 3, 2024 9:00:53 AM PDT, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>On 03/07/2024 4:54 pm, Borislav Petkov wrote:
>> Dave Hansen <dave.hansen@intel.com> wrote:
>>> On 7/3/24 01:54, Xin Li (Intel) wrote:
>>> &gt; Do FRED MSR writes with wrmsrns() rather than wrmsrl().
>>>
>>> A longer changelog would be appreciated here. The wrmsrns() is
>>> presumably to avoid the WRMSR serialization overhead and the CR4 write
>>> provides all of the serialization that we need.
>> Also, all those wrmsrns() writes better be behind a CPUID check.
>
>They're not, in Linux.
>
>For the $N'th time, here is the primitive that Linux wants to stea^w
>borrow for this to be sane.
>
>/* Non-serialising WRMSR, when available.  Falls back to a serialising
>WRMSR. */
>static inline void wrmsrns(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));
>}
>
>~Andrew

I believe tglx declared to use them unconditionally since FRED depends on WRMSRNS (and the kernel enforces that.)

Using an alternative would make wrmsrns() a more useful construct in general, though.