Re: [PATCH 6/6] x86/spec-ctrl: Introduce and use DO_COND_BHB_SEQ

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.04.2024 20:14, Andrew Cooper wrote:
> --- a/xen/arch/x86/hvm/vmx/entry.S
> +++ b/xen/arch/x86/hvm/vmx/entry.S
> @@ -62,12 +62,12 @@ ENTRY(vmx_asm_vmexit_handler)
>           * Clear the BHB to mitigate BHI.  Used on eIBRS parts, and uses RETs
>           * itself so must be after we've perfomed all the RET-safety we can.
>           */
> -        testb $SCF_entry_bhb, CPUINFO_scf(%rsp)
> -        jz .L_skip_bhb
> -        ALTERNATIVE_2 "",                                    \
> -            "call clear_bhb_loops", X86_SPEC_BHB_LOOPS,      \
> -            "call clear_bhb_tsx", X86_SPEC_BHB_TSX
> -.L_skip_bhb:
> +        .macro VMX_BHB_SEQ fn:req
> +            DO_COND_BHB_SEQ \fn scf=CPUINFO_scf(%rsp)
> +        .endm
> +        ALTERNATIVE_2 "",                                         \
> +            "VMX_BHB_SEQ fn=clear_bhb_loops", X86_SPEC_BHB_LOOPS, \
> +            "VMX_BHB_SEQ fn=clear_bhb_tsx",   X86_SPEC_BHB_TSX

Oh, and just to mention it since we were discussing this before: The variant
of this that I had been thinking of without decode-lite would have been to
transform this (readable)

        testb $SCF_entry_bhb, CPUINFO_scf(%rsp)
        ALTERNATIVE_2 "jmp .L_skip_bhb",                     \
            "jz .L_skip_bhb", X86_SPEC_BHB_LOOPS,            \
            "jz .L_skip_bhb", X86_SPEC_BHB_TSX
        ALTERNATIVE_2 "",                                    \
            "call clear_bhb_loops", X86_SPEC_BHB_LOOPS,      \
            "call clear_bhb_tsx", X86_SPEC_BHB_TSX
.L_skip_bhb:

into (untested, and hence perhaps slightly off) the (less readable)

        testb $SCF_entry_bhb, CPUINFO_scf(%rsp)
        ALTERNATIVE_2 ".byte 0xeb" /* jmp */,                \
            ".byte 0x74" /* jz */, X86_SPEC_BHB_LOOPS,       \
            ".byte 0x74" /* jz */, X86_SPEC_BHB_TSX
        .byte .L_skip_bhb - (. + 1)
        ALTERNATIVE_2 "",                                    \
            "call clear_bhb_loops", X86_SPEC_BHB_LOOPS,      \
            "call clear_bhb_tsx", X86_SPEC_BHB_TSX
.L_skip_bhb:

Of course yours (dropping the branch altogether) is better, but also comes
at a higher price.

Jan