[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 1/3] x86: allow Kconfig control over psABI level
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 > >
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