Re: [PATCH 1/3] x86: allow Kconfig control over psABI level

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.07.2023 14:33, Jan Beulich wrote:
> Newer hardware offers more efficient and/or flexible and/or capable
> instructions, some of which we can make good use of in the hypervisor
> as well. Allow a basic way (no alternatives patching) of enabling their
> use. Of course this means that hypervisors thus built won't work
> anymore on older, less capable hardware.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> TBD: Should we, just like for NX, add an early check in assembly code,
>      to have a "clean" failure rather than a random crash?

TBD: While older compilers (apparently gcc10 and older; not sure about
     clang) won't recognize -march=x86-64-v2 etc, we could fall back to
     passing -mpopcnt and alike explicitly.

Jan

> Whereas the baseline -> v2 step isn't much of a difference (we'll gain
> more there by a subsequent patch), v2 -> v3, while presumably (or shall
> I say hopefully) faster, yields an overall growth of .text size by (in
> my build) about 2k. The primary reason for this appear to be conversions
> of SHL-by-immediate to SHLX.
> 
> --- a/xen/arch/x86/Kconfig
> +++ b/xen/arch/x86/Kconfig
> @@ -118,6 +118,36 @@ config HVM
>  
>         If unsure, say Y.
>  
> +choice
> +     prompt "base psABI level"
> +     default X86_64_BASELINE
> +     help
> +       The psABI defines 4 levels of ISA extension sets as a coarse granular
> +       way of identifying advanced functionality that would be uniformly
> +       available in respectively newer hardware.  While v4 is not really of
> +       interest for Xen, the others can be selected here, making the
> +       resulting Xen no longer work on older hardware.  This option won't
> +       have any effect if the toolchain doesn't support the distinction.
> +
> +       If unsure, stick to the default.
> +
> +config X86_64_BASELINE
> +     bool "baseline"
> +
> +config X86_64_V2
> +     bool "v2"
> +     help
> +       This enables POPCNT and CX16, besides other extensions which are of
> +       no interest here.
> +
> +config X86_64_V3
> +     bool "v3"
> +     help
> +       This enables BMI, BMI2, LZCNT, and MOVBE, besides other extensions
> +       which are of no interest here.
> +
> +endchoice
> +
>  config XEN_SHSTK
>       bool "Supervisor Shadow Stacks"
>       depends on HAS_AS_CET_SS
> --- a/xen/arch/x86/arch.mk
> +++ b/xen/arch/x86/arch.mk
> @@ -36,6 +36,10 @@ CFLAGS += -mno-red-zone -fpic
>  # the SSE setup for variadic function calls.
>  CFLAGS += -mno-mmx -mno-sse $(call cc-option,$(CC),-mskip-rax-setup)
>  
> +# Enable the selected baseline ABI, if supported by the compiler.
> +CFLAGS-$(CONFIG_X86_64_V2) += $(call cc-option,$(CC),-march=x86-64-v2)
> +CFLAGS-$(CONFIG_X86_64_V3) += $(call cc-option,$(CC),-march=x86-64-v3)
> +
>  ifeq ($(CONFIG_INDIRECT_THUNK),y)
>  # Compile with gcc thunk-extern, indirect-branch-register if available.
>  CFLAGS-$(CONFIG_CC_IS_GCC) += -mindirect-branch=thunk-extern
> 
>