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 <[email protected]>
> ---
> 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
>
>
