[PATCH v4 0/3] x86: allow Kconfig control over psABI level

[PATCH v4 0/3] x86: allow Kconfig control over psABI level

[Jan Beulich]

As discussed in Prague.

v4 drops the apparently controversial patch "build: permit Kconfig control
over how to deal with unsatisfiable choices" and updates things now that
gcc5 is our baseline.

1: allow Kconfig control over psABI level
2: use POPCNT for hweight<N>() when available
3: short-circuit certain cpu_has_* when x86-64-v{2,3} are in effect

Jan

[PATCH v4 1/3] x86: allow Kconfig control over psABI level to use

[Jan Beulich]

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.

Since older compilers (apparently gcc10 / clang11 and older) won't
recognize -march=x86-64-v2 etc, also add fallback logic passing
-mpopcnt and alike explicitly.

Note that in efi_arch_cpu() the filling of 7c0 and 7d0 are forward-
looking; we only require 7b0, but we need to use cpuid_count() anyway.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
TBD: While the added assembly code goes strictly from the CONFIG_*
     settings, EFI code uses the compiler provided manifest constants
     where available (just like the subsequent "x86: short-circuit
     certain cpu_has_* when x86-64-v{2,3} are in effect"). While I
     generally prefer it that way, this comes with a downside: If we
     ever started to use one of the ISA extensions in assembly code
     (and then keyed to the CONFIG_* settings), things could break. I
     don't view us doing this as very likely though, as then we'd need
     to have two (or more) variants of such assembly code, which we
     would likely want to avoid. (What I'd like to avoid is using the
     compiler manifest constants in assembly code: In principle we ought
     to filter out any -march= when constructing AFLAGS, and perhaps
     at least all -m... and -f... options. Compilers might rightfully
     complain about their use as inapplicable, much like we've seen for
     -Wl,... when no linking is done.)

TBD: While we don't document most "cpuid=no-*" options (and hence imply
     their use to be unsupported), using e.g. "cpuid=no-popcnt" with a
     V2=y hypervisor clearly can't have the intended effect, and hence
     might perhaps better be flagged in some way.

TBD: v2 also includes LAHF/SAHF. Since we don't use floating point and
     hence FPU insns, we ought to be okay not explicitly checking for
     it. But there is a certain amount of risk of the compiler finding
     some "smart" use for one or both of the insns. However, if we were
     to check the feature, we'd need to account for the quirk that
     init_amd() also works around.

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.

The VGA output attempt in early (MB/MB2) boot code does not appear to
work (anymore?). The serial output may work, but only if - without any
setup in Xen - both sides agree on the serial settings (baud rate etc).
Hence the feature checks added on the legacy paths are of limited use.
---
v4: Correct __LZCNT__ check in efi_arch_cpu(). Drop CONFIG_UNSATISFIED_*
    part. Simplify logic in arch.mk, now that gcc5 is our baseline. Reduce
    code size in trampoline_setup. Re-base.
v3: Add fallback logic and use logic from new prereq change.
v2: Also cover XSAVE. Add early boot feature checking.

--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -116,6 +116,36 @@ config PV_LINEAR_PT
 
          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, MOVBE, and XSAVE, besides other
+	  extensions which are of no interest here.
+
+endchoice
+
 source "arch/x86/hvm/Kconfig"
 
 config XEN_SHSTK
--- a/xen/arch/x86/arch.mk
+++ b/xen/arch/x86/arch.mk
@@ -34,6 +34,27 @@ 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.
+x86-64-$(CONFIG_X86_64_BASELINE) :=
+x86-64-$(CONFIG_X86_64_V2) := x86-64-v2
+x86-64-$(CONFIG_X86_64_V3) := x86-64-v3
+
+ifneq ($(x86-64-y),)
+
+CFLAGS-psabi := $(call cc-option,$(CC),-march=$(x86-64-y))
+
+ifneq ($(CFLAGS-psabi),)
+CFLAGS += $(CFLAGS-psabi)
+else
+# Fall back to using the subset of -m... options that are of interest.
+x86-64-v2 := -mpopcnt -mcx16
+x86-64-v3 := $(x86-64-v2) -mbmi -mbmi2 -mlzcnt -mmovbe -mxsave
+CFLAGS-$(CONFIG_X86_64_V2) += $(x86-64-v2)
+CFLAGS-$(CONFIG_X86_64_V3) += $(x86-64-v3)
+endif # CFLAGS-psabi
+
+endif # x86-64-y
+
 ifeq ($(CONFIG_INDIRECT_THUNK),y)
 # Compile with gcc thunk-extern, indirect-branch-register if available.
 CFLAGS-$(CONFIG_CC_IS_GCC) += -mindirect-branch=thunk-extern
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -136,6 +136,12 @@ multiboot2_header:
 #ifdef CONFIG_REQUIRE_NX
 .Lno_nx_msg:   .asciz "ERR: Not an NX-capable CPU!"
 #endif
+#if defined(CONFIG_X86_64_V2) || defined(CONFIG_X86_64_V3)
+.Lno_x86_64_v2_msg: .asciz "ERR: Not an x86-64-v2 capable CPU!"
+#endif
+#ifdef CONFIG_X86_64_V3
+.Lno_x86_64_v3_msg: .asciz "ERR: Not an x86-64-v3 capable CPU!"
+#endif
 
         .section .init.data, "aw", @progbits
         .subsection 1 /* Put data here after the page tables (in x86_64.S). */
@@ -169,9 +175,20 @@ early_error: /* Here to improve the disa
         jmp     .Lget_vtb
 #ifdef CONFIG_REQUIRE_NX
 .Lno_nx:
+        pop     %ecx
         mov     $sym_offs(.Lno_nx_msg), %ecx
         jmp     .Lget_vtb
 #endif
+#if defined(CONFIG_X86_64_V2) || defined(CONFIG_X86_64_V3)
+.Lno_x86_64_v2:
+        mov     $sym_offs(.Lno_x86_64_v2_msg), %ecx
+        jmp     .Lget_vtb
+#endif
+#ifdef CONFIG_X86_64_V3
+.Lno_x86_64_v3:
+        mov     $sym_offs(.Lno_x86_64_v3_msg), %ecx
+        jmp     .Lget_vtb
+#endif
 .Lmb2_no_bs:
         /*
          * Ditto. Additionally, here there is a chance that Xen was started
@@ -532,6 +549,7 @@ trampoline_setup:
         mov     $1, %eax
         cpuid
         mov     %ecx, CPUINFO_FEATURE_OFFSET(X86_FEATURE_HYPERVISOR) + sym_esi(boot_cpu_data)
+        mov     %ecx, %edi
 
         mov     $0x80000000,%eax
         cpuid
@@ -549,6 +567,9 @@ trampoline_setup:
         bt      $cpufeat_bit(X86_FEATURE_LM),%edx
         jnc     .Lbad_cpu
 
+        /* Preserve %ecx for later use. */
+        push    %ecx
+
         /*
          * Check for NX
          *   - If Xen was compiled requiring it simply assert it's
@@ -599,6 +620,36 @@ trampoline_setup:
 .Lno_nx:
 #endif
 
+        /* Restore CPUID[80000001].ecx output. */
+        pop     %ecx
+
+#if defined(CONFIG_X86_64_V2) || defined(CONFIG_X86_64_V3)
+        mov     %edi, %eax
+        or      $cpufeat_mask(X86_FEATURE_POPCNT) | cpufeat_mask(X86_FEATURE_CX16), %eax
+        cmp     %edi, %eax
+        jne     .Lno_x86_64_v2
+#endif
+
+#ifdef CONFIG_X86_64_V3
+        mov     %edi, %eax
+        or      $cpufeat_mask(X86_FEATURE_MOVBE) | cpufeat_mask(X86_FEATURE_XSAVE), %eax
+        cmp     %edi, %eax
+        jne     .Lno_x86_64_v3
+        bt      $cpufeat_bit(X86_FEATURE_ABM), %ecx
+        jnc     .Lno_x86_64_v3
+        xor     %eax, %eax
+        cpuid
+        cmp     $7, %eax
+        jb      .Lno_x86_64_v3
+        mov     $7, %eax
+        xor     %ecx, %ecx
+        cpuid
+        mov     %ebx, %eax
+        or      $cpufeat_mask(X86_FEATURE_BMI1) | cpufeat_mask(X86_FEATURE_BMI2), %eax
+        cmp     %ebx, %eax
+        jne     .Lno_x86_64_v3
+#endif
+
         /* Stash TSC to calculate a good approximation of time-since-boot */
         rdtsc
         mov     %eax,     sym_esi(boot_tsc_stamp)
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -742,29 +742,53 @@ static void __init efi_arch_handle_modul
 
 static void __init efi_arch_cpu(void)
 {
-    uint32_t eax = cpuid_eax(0x80000000U);
+    uint32_t eax = cpuid_eax(0), dummy;
     uint32_t *caps = boot_cpu_data.x86_capability;
 
     boot_tsc_stamp = rdtsc();
 
-    caps[FEATURESET_1c] = cpuid_ecx(1);
+    if ( eax )
+        caps[FEATURESET_1c] = cpuid_ecx(1);
 
+    if ( eax >= 7 )
+        cpuid_count(7, 0, &dummy,
+                    &caps[FEATURESET_7b0],
+                    &caps[FEATURESET_7c0],
+                    &caps[FEATURESET_7d0]);
+
+    eax = cpuid_eax(0x80000000U);
     if ( (eax >> 16) == 0x8000 && eax > 0x80000000U )
-    {
-        caps[FEATURESET_e1d] = cpuid_edx(0x80000001U);
+        cpuid(0x80000001U, &dummy, &dummy,
+              &caps[FEATURESET_e1c], &caps[FEATURESET_e1d]);
+
+    /*
+     * Checks here purposefully don't use cpu_has_* because they bypass
+     * boot_cpu_data reads if Xen was compiled with respective CONFIG_*.
+     */
+#define CHK(ppsym, feat) do {                                        \
+        if ( IS_ENABLED(ppsym) &&                                    \
+             !boot_cpu_has(X86_FEATURE_ ## feat) )                   \
+            blexit(L"This build of Xen requires " #feat " support"); \
+    } while ( false )
+
+    CHK(CONFIG_REQUIRE_NX, NX);
+
+    /* x86-64-v2 */
+    CHK(__POPCNT__, POPCNT);
+    CHK(CONFIG_X86_64_V2, CX16);
+    CHK(CONFIG_X86_64_V3, CX16);
+
+    /* x86-64-v3 */
+    CHK(__LZCNT__, ABM);
+    CHK(__BMI__, BMI1);
+    CHK(__BMI2__, BMI2);
+    CHK(__MOVBE__, MOVBE);
+    CHK(__XSAVE__, XSAVE);
+
+#undef CHK
 
-        /*
-         * This check purposefully doesn't use cpu_has_nx because
-         * cpu_has_nx bypasses the boot_cpu_data read if Xen was compiled
-         * with CONFIG_REQUIRE_NX
-         */
-        if ( IS_ENABLED(CONFIG_REQUIRE_NX) &&
-             !boot_cpu_has(X86_FEATURE_NX) )
-            blexit(L"This build of Xen requires NX support");
-
-        if ( cpu_has_nx )
-            trampoline_efer |= EFER_NXE;
-    }
+    if ( cpu_has_nx )
+        trampoline_efer |= EFER_NXE;
 }
 
 static void __init efi_arch_blexit(void)

[PATCH v4 2/3] x86: use __builtin_popcountl() for arch_hweightl() when available

[Jan Beulich]

Use the respective compiler builtin when available; no need to play any
alternatives games in such configurations.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v4: Re-base (re-do).

--- a/xen/arch/x86/include/asm/bitops.h
+++ b/xen/arch/x86/include/asm/bitops.h
@@ -500,6 +500,9 @@ static always_inline unsigned int arch_f
 
 unsigned int arch_generic_hweightl(unsigned long x);
 
+#ifdef __POPCNT__
+#define arch_hweightl __builtin_popcountl
+#else /* !__POPCNT__ */
 static always_inline unsigned int arch_hweightl(unsigned long x)
 {
     unsigned int r;
@@ -525,5 +528,6 @@ static always_inline unsigned int arch_h
     return r;
 }
 #define arch_hweightl arch_hweightl
+#endif /* __POPCNT__ */
 
 #endif /* _X86_BITOPS_H */

Re: [PATCH v4 2/3] x86: use __builtin_popcountl() for arch_hweightl() when available

[Andrew Cooper]

On 15/10/2026 10:10 am, Jan Beulich wrote:
> Use the respective compiler builtin when available; no need to play any
> alternatives games in such configurations.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> v4: Re-base (re-do).
>
> --- a/xen/arch/x86/include/asm/bitops.h
> +++ b/xen/arch/x86/include/asm/bitops.h
> @@ -500,6 +500,9 @@ static always_inline unsigned int arch_f
>  
>  unsigned int arch_generic_hweightl(unsigned long x);
>  
> +#ifdef __POPCNT__
> +#define arch_hweightl __builtin_popcountl
> +#else /* !__POPCNT__ */
>  static always_inline unsigned int arch_hweightl(unsigned long x)
>  {
>      unsigned int r;
> @@ -525,5 +528,6 @@ static always_inline unsigned int arch_h
>      return r;
>  }
>  #define arch_hweightl arch_hweightl
> +#endif /* __POPCNT__ */
>  
>  #endif /* _X86_BITOPS_H */
>

This is needlessly complex.

diff --git a/xen/arch/x86/include/asm/bitops.h
b/xen/arch/x86/include/asm/bitops.h
index 557cee03baeb..c38680e8888c 100644
--- a/xen/arch/x86/include/asm/bitops.h
+++ b/xen/arch/x86/include/asm/bitops.h
@@ -504,6 +504,10 @@ static always_inline unsigned int
arch_hweightl(unsigned long x)
 {
     unsigned int r;
 
+    /* If compiled with a suitable baseline ISA. */
+    if ( IS_ENABLED(__POPCNT__) )
+        return __builtin_popcountl(x);
+
     /*
      * arch_generic_hweightl() is written in ASM in order to preserve all
      * registers, as the compiler can't see the call.


~Andrew

Re: [PATCH v4 2/3] x86: use __builtin_popcountl() for arch_hweightl() when available

[Jan Beulich]

On 15.10.2026 12:37, Andrew Cooper wrote:
> On 15/10/2026 10:10 am, Jan Beulich wrote:
>> Use the respective compiler builtin when available; no need to play any
>> alternatives games in such configurations.
>>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>> ---
>> v4: Re-base (re-do).
>>
>> --- a/xen/arch/x86/include/asm/bitops.h
>> +++ b/xen/arch/x86/include/asm/bitops.h
>> @@ -500,6 +500,9 @@ static always_inline unsigned int arch_f
>>  
>>  unsigned int arch_generic_hweightl(unsigned long x);
>>  
>> +#ifdef __POPCNT__
>> +#define arch_hweightl __builtin_popcountl
>> +#else /* !__POPCNT__ */
>>  static always_inline unsigned int arch_hweightl(unsigned long x)
>>  {
>>      unsigned int r;
>> @@ -525,5 +528,6 @@ static always_inline unsigned int arch_h
>>      return r;
>>  }
>>  #define arch_hweightl arch_hweightl
>> +#endif /* __POPCNT__ */
>>  
>>  #endif /* _X86_BITOPS_H */
>>
> 
> This is needlessly complex.

I don't mind doing it this way. (It's less complex in source then, but more
complex for the compiler.)

Jan

> --- a/xen/arch/x86/include/asm/bitops.h
> +++ b/xen/arch/x86/include/asm/bitops.h
> @@ -504,6 +504,10 @@ static always_inline unsigned int
> arch_hweightl(unsigned long x)
>  {
>      unsigned int r;
>  
> +    /* If compiled with a suitable baseline ISA. */
> +    if ( IS_ENABLED(__POPCNT__) )
> +        return __builtin_popcountl(x);
> +
>      /*
>       * arch_generic_hweightl() is written in ASM in order to preserve all
>       * registers, as the compiler can't see the call.
> 
> 
> ~Andrew

[PATCH v4 3/3] x86: short-circuit certain cpu_has_* when x86-64-v{2,3} are in effect

[Jan Beulich]

Certain fallback code can be made subject to DCE this way. Note that
CX16 has no compiler provided manifest constant, so CONFIG_* are used
there instead. Note also that we don't have cpu_has_movbe nor
cpu_has_lzcnt (aka cpu_has_abm), and we no longer have
cpu_has_{popcnt,bmi1,bmi2}.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
---
Of course we could use IS_ENABLED(CONFIG_X86_64_V<n>) everywhere, but as
CX16 shows this isn't necessarily better than the #if/#else approach
based on compiler-provided manifest symbols. While not really intended
to be used that way, it looks as if we could also use
IS_ENABLED(__POPCNT__) and alike if we thought this would end up neater
(because of avoiding the #ifdef).

We could go further and also short-circuit SSE*, AVX and alike, which we
don't use outside of the emulator. This then would of course call for
also having a way to select x86-64-v4.
---
v4: Re-base (over dropped cpu_has_{popcnt,bmi1,bmi2}).
v2: Also cover XSAVE.

--- a/xen/arch/x86/include/asm/cpufeature.h
+++ b/xen/arch/x86/include/asm/cpufeature.h
@@ -132,11 +132,17 @@ static inline bool boot_cpu_has(unsigned
 #define cpu_has_monitor         boot_cpu_has(X86_FEATURE_MONITOR)
 #define cpu_has_vmx             boot_cpu_has(X86_FEATURE_VMX)
 #define cpu_has_eist            boot_cpu_has(X86_FEATURE_EIST)
-#define cpu_has_cx16            boot_cpu_has(X86_FEATURE_CX16)
+#define cpu_has_cx16            (IS_ENABLED(CONFIG_X86_64_V2) || \
+                                 IS_ENABLED(CONFIG_X86_64_V3) || \
+                                 boot_cpu_has(X86_FEATURE_CX16))
 #define cpu_has_pdcm            boot_cpu_has(X86_FEATURE_PDCM)
 #define cpu_has_pcid            boot_cpu_has(X86_FEATURE_PCID)
 #define cpu_has_x2apic          boot_cpu_has(X86_FEATURE_X2APIC)
+#ifdef __XSAVE__
+#define cpu_has_xsave           true
+#else
 #define cpu_has_xsave           boot_cpu_has(X86_FEATURE_XSAVE)
+#endif
 #define cpu_has_avx             boot_cpu_has(X86_FEATURE_AVX)
 #define cpu_has_rdrand          boot_cpu_has(X86_FEATURE_RDRAND)
 #define cpu_has_hypervisor      boot_cpu_has(X86_FEATURE_HYPERVISOR)

Re: [PATCH v4 3/3] x86: short-circuit certain cpu_has_* when x86-64-v{2,3} are in effect

[Roger Pau Monné]

On Thu, Oct 15, 2026 at 11:11:39AM +0200, Jan Beulich wrote:
> Certain fallback code can be made subject to DCE this way. Note that
> CX16 has no compiler provided manifest constant, so CONFIG_* are used
> there instead. Note also that we don't have cpu_has_movbe nor
> cpu_has_lzcnt (aka cpu_has_abm), and we no longer have
> cpu_has_{popcnt,bmi1,bmi2}.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
> ---
> Of course we could use IS_ENABLED(CONFIG_X86_64_V<n>) everywhere, but as
> CX16 shows this isn't necessarily better than the #if/#else approach
> based on compiler-provided manifest symbols. While not really intended
> to be used that way, it looks as if we could also use
> IS_ENABLED(__POPCNT__) and alike if we thought this would end up neater
> (because of avoiding the #ifdef).
> 
> We could go further and also short-circuit SSE*, AVX and alike, which we
> don't use outside of the emulator. This then would of course call for
> also having a way to select x86-64-v4.
> ---
> v4: Re-base (over dropped cpu_has_{popcnt,bmi1,bmi2}).
> v2: Also cover XSAVE.
> 
> --- a/xen/arch/x86/include/asm/cpufeature.h
> +++ b/xen/arch/x86/include/asm/cpufeature.h
> @@ -132,11 +132,17 @@ static inline bool boot_cpu_has(unsigned
>  #define cpu_has_monitor         boot_cpu_has(X86_FEATURE_MONITOR)
>  #define cpu_has_vmx             boot_cpu_has(X86_FEATURE_VMX)
>  #define cpu_has_eist            boot_cpu_has(X86_FEATURE_EIST)
> -#define cpu_has_cx16            boot_cpu_has(X86_FEATURE_CX16)
> +#define cpu_has_cx16            (IS_ENABLED(CONFIG_X86_64_V2) || \
> +                                 IS_ENABLED(CONFIG_X86_64_V3) || \
> +                                 boot_cpu_has(X86_FEATURE_CX16))
>  #define cpu_has_pdcm            boot_cpu_has(X86_FEATURE_PDCM)
>  #define cpu_has_pcid            boot_cpu_has(X86_FEATURE_PCID)
>  #define cpu_has_x2apic          boot_cpu_has(X86_FEATURE_X2APIC)
> +#ifdef __XSAVE__
> +#define cpu_has_xsave           true
> +#else
>  #define cpu_has_xsave           boot_cpu_has(X86_FEATURE_XSAVE)
> +#endif
>  #define cpu_has_avx             boot_cpu_has(X86_FEATURE_AVX)
>  #define cpu_has_rdrand          boot_cpu_has(X86_FEATURE_RDRAND)
>  #define cpu_has_hypervisor      boot_cpu_has(X86_FEATURE_HYPERVISOR)

Alejandro (now added on Cc) is working on something that achieves the
same end, but it's more transparent and doesn't require modifying
individual cpu_has_ macros every time we want to short-circuit one of
those.  I think it would be helpful here also.

Regards, Roger.

Re: [PATCH v4 3/3] x86: short-circuit certain cpu_has_* when x86-64-v{2,3} are in effect

[Alejandro Vallejo]

On Mon Oct 19, 2026 at 11:34 AM CEST, Roger Pau Monné wrote:
> On Thu, Oct 15, 2026 at 11:11:39AM +0200, Jan Beulich wrote:
> > Certain fallback code can be made subject to DCE this way. Note that
> > CX16 has no compiler provided manifest constant, so CONFIG_* are used
> > there instead. Note also that we don't have cpu_has_movbe nor
> > cpu_has_lzcnt (aka cpu_has_abm), and we no longer have
> > cpu_has_{popcnt,bmi1,bmi2}.
> > 
> > Signed-off-by: Jan Beulich <jbeulich@suse.com>
> > Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
> > ---
> > Of course we could use IS_ENABLED(CONFIG_X86_64_V<n>) everywhere, but as
> > CX16 shows this isn't necessarily better than the #if/#else approach
> > based on compiler-provided manifest symbols. While not really intended
> > to be used that way, it looks as if we could also use
> > IS_ENABLED(__POPCNT__) and alike if we thought this would end up neater
> > (because of avoiding the #ifdef).
> > 
> > We could go further and also short-circuit SSE*, AVX and alike, which we
> > don't use outside of the emulator. This then would of course call for
> > also having a way to select x86-64-v4.
> > ---
> > v4: Re-base (over dropped cpu_has_{popcnt,bmi1,bmi2}).
> > v2: Also cover XSAVE.
> > 
> > --- a/xen/arch/x86/include/asm/cpufeature.h
> > +++ b/xen/arch/x86/include/asm/cpufeature.h
> > @@ -132,11 +132,17 @@ static inline bool boot_cpu_has(unsigned
> >  #define cpu_has_monitor         boot_cpu_has(X86_FEATURE_MONITOR)
> >  #define cpu_has_vmx             boot_cpu_has(X86_FEATURE_VMX)
> >  #define cpu_has_eist            boot_cpu_has(X86_FEATURE_EIST)
> > -#define cpu_has_cx16            boot_cpu_has(X86_FEATURE_CX16)
> > +#define cpu_has_cx16            (IS_ENABLED(CONFIG_X86_64_V2) || \
> > +                                 IS_ENABLED(CONFIG_X86_64_V3) || \
> > +                                 boot_cpu_has(X86_FEATURE_CX16))
> >  #define cpu_has_pdcm            boot_cpu_has(X86_FEATURE_PDCM)
> >  #define cpu_has_pcid            boot_cpu_has(X86_FEATURE_PCID)
> >  #define cpu_has_x2apic          boot_cpu_has(X86_FEATURE_X2APIC)
> > +#ifdef __XSAVE__
> > +#define cpu_has_xsave           true
> > +#else
> >  #define cpu_has_xsave           boot_cpu_has(X86_FEATURE_XSAVE)
> > +#endif
> >  #define cpu_has_avx             boot_cpu_has(X86_FEATURE_AVX)
> >  #define cpu_has_rdrand          boot_cpu_has(X86_FEATURE_RDRAND)
> >  #define cpu_has_hypervisor      boot_cpu_has(X86_FEATURE_HYPERVISOR)
>
> Alejandro (now added on Cc) is working on something that achieves the
> same end, but it's more transparent and doesn't require modifying
> individual cpu_has_ macros every time we want to short-circuit one of
> those.  I think it would be helpful here also.

Yes, my cpu_has work would simplify this quite a lot, and it would
effectively deprecate the ad-hoc nature of REQUIRE_NX cont-ification.

I'll try sending upstream an adapted version some time early this week.

Cheers,
Alejandro

>
> Regards, Roger.