[PATCH v2 1/3] x86: Use MOVL when reading segment registers

[PATCH v2 1/3] x86: Use MOVL when reading segment registers

[Uros Bizjak]

Use MOVL when reading segment registers to avoid 0x66 operand-size
override insn prefix. The segment value is always 16-bit and gets
zero-extended to the full 32-bit size.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
 arch/x86/include/asm/segment.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index f59ae7186940..9f5be2bbd291 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -348,7 +348,7 @@ static inline void __loadsegment_fs(unsigned short value)
  * Save a segment register away:
  */
 #define savesegment(seg, value)				\
-	asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
+	asm("movl %%" #seg ",%k0" : "=r" (value) : : "memory")
 
 #endif /* !__ASSEMBLER__ */
 #endif /* __KERNEL__ */
-- 
2.51.1

[PATCH v2 2/3] x86/hyperv: Use savesegment() instead of inline asm() to save segment registers

[Uros Bizjak]

Use standard savesegment() utility macro to save segment registers.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
 arch/x86/hyperv/ivm.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
index 651771534cae..7365d8f43181 100644
--- a/arch/x86/hyperv/ivm.c
+++ b/arch/x86/hyperv/ivm.c
@@ -25,6 +25,7 @@
 #include <asm/e820/api.h>
 #include <asm/desc.h>
 #include <asm/msr.h>
+#include <asm/segment.h>
 #include <uapi/asm/vmx.h>
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT
@@ -315,16 +316,16 @@ int hv_snp_boot_ap(u32 apic_id, unsigned long start_ip, unsigned int cpu)
 	vmsa->gdtr.base = gdtr.address;
 	vmsa->gdtr.limit = gdtr.size;
 
-	asm volatile("movl %%es, %%eax;" : "=a" (vmsa->es.selector));
+	savesegment(es, vmsa->es.selector);
 	hv_populate_vmcb_seg(vmsa->es, vmsa->gdtr.base);
 
-	asm volatile("movl %%cs, %%eax;" : "=a" (vmsa->cs.selector));
+	savesegment(cs, vmsa->cs.selector);
 	hv_populate_vmcb_seg(vmsa->cs, vmsa->gdtr.base);
 
-	asm volatile("movl %%ss, %%eax;" : "=a" (vmsa->ss.selector));
+	savesegment(ss, vmsa->ss.selector);
 	hv_populate_vmcb_seg(vmsa->ss, vmsa->gdtr.base);
 
-	asm volatile("movl %%ds, %%eax;" : "=a" (vmsa->ds.selector));
+	savesegment(ds, vmsa->ds.selector);
 	hv_populate_vmcb_seg(vmsa->ds, vmsa->gdtr.base);
 
 	vmsa->efer = native_read_msr(MSR_EFER);
-- 
2.51.1

[PATCH v2 3/3] x86/hyperv: Remove ASM_CALL_CONSTRAINT with VMMCALL insn

[Uros Bizjak]

Unlike CALL instruction, VMMCALL does not push to the stack, so it's
OK to allow the compiler to insert it before the frame pointer gets
set up by the containing function. ASM_CALL_CONSTRAINT is for CALLs
that must be inserted after the frame pointer is set up, so it is
over-constraining here and can be removed.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
v2: Expand commit message and include ASM_CALL_CONSTRAINT explanation
---
 arch/x86/hyperv/ivm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
index 7365d8f43181..be7fad43a88d 100644
--- a/arch/x86/hyperv/ivm.c
+++ b/arch/x86/hyperv/ivm.c
@@ -392,7 +392,7 @@ u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2)
 
 	register u64 __r8 asm("r8") = param2;
 	asm volatile("vmmcall"
-		     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
+		     : "=a" (hv_status),
 		       "+c" (control), "+d" (param1), "+r" (__r8)
 		     : : "cc", "memory", "r9", "r10", "r11");
 
-- 
2.51.1

Re: [PATCH v2 3/3] x86/hyperv: Remove ASM_CALL_CONSTRAINT with VMMCALL insn

[H. Peter Anvin]

On November 21, 2025 6:14:11 AM PST, Uros Bizjak <ubizjak@gmail.com> wrote:
>Unlike CALL instruction, VMMCALL does not push to the stack, so it's
>OK to allow the compiler to insert it before the frame pointer gets
>set up by the containing function. ASM_CALL_CONSTRAINT is for CALLs
>that must be inserted after the frame pointer is set up, so it is
>over-constraining here and can be removed.
>
>Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
>Tested-by: Michael Kelley <mhklinux@outlook.com>
>Cc: "K. Y. Srinivasan" <kys@microsoft.com>
>Cc: Haiyang Zhang <haiyangz@microsoft.com>
>Cc: Wei Liu <wei.liu@kernel.org>
>Cc: Dexuan Cui <decui@microsoft.com>
>Cc: Thomas Gleixner <tglx@linutronix.de>
>Cc: Ingo Molnar <mingo@redhat.com>
>Cc: Borislav Petkov <bp@alien8.de>
>Cc: Dave Hansen <dave.hansen@linux.intel.com>
>Cc: "H. Peter Anvin" <hpa@zytor.com>
>---
>v2: Expand commit message and include ASM_CALL_CONSTRAINT explanation
>---
> arch/x86/hyperv/ivm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
>index 7365d8f43181..be7fad43a88d 100644
>--- a/arch/x86/hyperv/ivm.c
>+++ b/arch/x86/hyperv/ivm.c
>@@ -392,7 +392,7 @@ u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2)
> 
> 	register u64 __r8 asm("r8") = param2;
> 	asm volatile("vmmcall"
>-		     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
>+		     : "=a" (hv_status),
> 		       "+c" (control), "+d" (param1), "+r" (__r8)
> 		     : : "cc", "memory", "r9", "r10", "r11");
> 

I think it would be good to have a comment at the point where ASM_CALL_CONSTRAINT is defined explaining its proper use.

Specifically, instructions like syscall, vmcall, vmfunc, vmmcall, int xx and VM-specific escape instructions are not "calls" because they either don't modify the stack or create an exception frame (kernel) or signal frame (user space) which is completely special.

Re: [PATCH v2 3/3] x86/hyperv: Remove ASM_CALL_CONSTRAINT with VMMCALL insn

[Uros Bizjak]

On Sat, Nov 22, 2025 at 1:06 AM H. Peter Anvin <hpa@zytor.com> wrote:
>
> On November 21, 2025 6:14:11 AM PST, Uros Bizjak <ubizjak@gmail.com> wrote:
> >Unlike CALL instruction, VMMCALL does not push to the stack, so it's
> >OK to allow the compiler to insert it before the frame pointer gets
> >set up by the containing function. ASM_CALL_CONSTRAINT is for CALLs
> >that must be inserted after the frame pointer is set up, so it is
> >over-constraining here and can be removed.
> >
> >Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> >Tested-by: Michael Kelley <mhklinux@outlook.com>
> >Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> >Cc: Haiyang Zhang <haiyangz@microsoft.com>
> >Cc: Wei Liu <wei.liu@kernel.org>
> >Cc: Dexuan Cui <decui@microsoft.com>
> >Cc: Thomas Gleixner <tglx@linutronix.de>
> >Cc: Ingo Molnar <mingo@redhat.com>
> >Cc: Borislav Petkov <bp@alien8.de>
> >Cc: Dave Hansen <dave.hansen@linux.intel.com>
> >Cc: "H. Peter Anvin" <hpa@zytor.com>
> >---
> >v2: Expand commit message and include ASM_CALL_CONSTRAINT explanation
> >---
> > arch/x86/hyperv/ivm.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
> >index 7365d8f43181..be7fad43a88d 100644
> >--- a/arch/x86/hyperv/ivm.c
> >+++ b/arch/x86/hyperv/ivm.c
> >@@ -392,7 +392,7 @@ u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2)
> >
> >       register u64 __r8 asm("r8") = param2;
> >       asm volatile("vmmcall"
> >-                   : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> >+                   : "=a" (hv_status),
> >                      "+c" (control), "+d" (param1), "+r" (__r8)
> >                    : : "cc", "memory", "r9", "r10", "r11");
> >
>
> I think it would be good to have a comment at the point where ASM_CALL_CONSTRAINT is defined explaining its proper use.
>
> Specifically, instructions like syscall, vmcall, vmfunc, vmmcall, int xx and VM-specific escape instructions are not "calls" because they either don't modify the stack or create an exception frame (kernel) or signal frame (user space) which is completely special.

The existing comment already mentions CALL instruction only:

/*
 * This output constraint should be used for any inline asm which has a "call"
 * instruction.  Otherwise the asm may be inserted before the frame pointer
 * gets set up by the containing function.  If you forget to do this, objtool
 * may print a "call without frame pointer save/setup" warning.
 */

Uros.