Re: [PATCH] x86: prefer RDTSCP in rdtsc_ordered()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/09/2024 4:08 pm, Jan Beulich wrote:
> If available, its use is supposed to be cheaper than LFENCE+RDTSC, and
> is virtually guaranteed to be cheaper than MFENCE+RDTSC.
>
> Unlike in rdtsc() use 64-bit local variables, eliminating the need for

I'd drop this reference to rdtsc() seeing as you adjust it in a parallel
patch.

> the compiler to emit a zero-extension insn for %eax (that's a cheap MOV,
> yet still pointless to have).
>
> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/arch/x86/include/asm/msr.h
> +++ b/xen/arch/x86/include/asm/msr.h
> @@ -108,18 +108,30 @@ static inline uint64_t rdtsc(void)
>  
>  static inline uint64_t rdtsc_ordered(void)
>  {
> -     /*
> -      * The RDTSC instruction is not ordered relative to memory access.
> -      * The Intel SDM and the AMD APM are both vague on this point, but
> -      * empirically an RDTSC instruction can be speculatively executed
> -      * before prior loads.  An RDTSC immediately after an appropriate
> -      * barrier appears to be ordered as a normal load, that is, it
> -      * provides the same ordering guarantees as reading from a global
> -      * memory location that some other imaginary CPU is updating
> -      * continuously with a time stamp.
> -      */
> -     alternative("lfence", "mfence", X86_FEATURE_MFENCE_RDTSC);
> -     return rdtsc();
> +    uint64_t low, high, aux;
> +
> +    /*
> +     * The RDTSC instruction is not ordered relative to memory access.
> +     * The Intel SDM and the AMD APM are both vague on this point, but
> +     * empirically an RDTSC instruction can be speculatively executed
> +     * before prior loads.

This part of the comment is stale now.  For RDTSC, AMD state:

"This instruction is not serializing. Therefore, there is no guarantee
that all instructions have completed at the time the time-stamp counter
is read."

and for RDTSCP:

"Unlike the RDTSC instruction, RDTSCP forces all older instructions to
retire before reading the time-stamp counter."

i.e. it's dispatch serialising, given our new post-Spectre terminology.


Intel OTOH have much more extensive information.  For RDTSC:

The RDTSC instruction is not a serializing instruction. It does not
necessarily wait until all previous instructions have been executed
before reading the counter. Similarly, subsequent instructions may begin
execution before the read operation is performed. The following items
may guide software seeking to order executions of RDTSC:

•If software requires RDTSC to be executed only after all previous
instructions have executed and all previous loads are globally visible,1
it can execute LFENCE immediately before RDTSC.

•If software requires RDTSC to be executed only after all previous
instructions have executed and all previous loads and stores are
globally visible, it can execute the sequence MFENCE;LFENCE immediately
before RDTSC.

•If software requires RDTSC to be executed prior to execution of any
subsequent instruction (including any memory accesses), it can execute
the sequence LFENCE immediately after RDTSC.

Similarly, for RDTSCP:

The RDTSCP instruction is not a serializing instruction, but it does
wait until all previous instructions have executed and all previous
loads are globally visible.  But it does not wait for previous stores to
be globally visible, and subsequent instructions may begin execution
before the read operation is performed. The following items may guide
software seeking to order executions of RDTSCP:

•If software requires RDTSCP to be executed only after all previous
stores are globally visible, it can execute MFENCE immediately before
RDTSCP.

•If software requires RDTSCP to be executed prior to execution of any
subsequent instruction (including any memory accesses), it can execute
LFENCE immediately after RDTSCP.



I'd delete most of the paragraph, and just state the recommendation to
use LFENCE.

In truth, X86_FEATURE_MFENCE_RDTSC is useless now that we unilaterally
activate LFENCE_DISPATCH on CPUs where it's optional.  Linux went as far
as removing the case entirely, because if you're running under a
hypervisor which hasn't set LFENCE_DISPATCH, then the misbehaviour of
lfence;rdtsc is the least of your problems.

~Andrew