[PATCH -hyperv 1/3] x86/hyperv: Save segment registers directly to memory in hv_hvcrash_ctxt_save()

[PATCH -hyperv 1/3] x86/hyperv: Save segment registers directly to memory in hv_hvcrash_ctxt_save()

[Uros Bizjak]

hv_hvcrash_ctxt_save() in arch/x86/hyperv/hv_crash.c currently saves
segment registers via a general-purpose register (%eax). Update the
code to save segment registers (cs, ss, ds, es, fs, gs) directly to
the crash context memory using movw. This avoids unnecessary use of
a general-purpose register, making the code simpler and more efficient.

The size of the corresponding object file improves as follows:

   text    data     bss     dec     hex filename
   4167     176     200    4543    11bf hv_crash-old.o
   4151     176     200    4527    11af hv_crash-new.o

No functional change occurs to the saved context contents; this is
purely a code-quality improvement.

Signed-off-by: Uros Bizjak <ubizjak@gmail.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: Long Li <longli@microsoft.com>
Cc: Thomas Gleixner <tglx@kernel.org>
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/hyperv/hv_crash.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/hyperv/hv_crash.c b/arch/x86/hyperv/hv_crash.c
index fdb277bf73d8..2c7ea7e70854 100644
--- a/arch/x86/hyperv/hv_crash.c
+++ b/arch/x86/hyperv/hv_crash.c
@@ -207,12 +207,12 @@ static void hv_hvcrash_ctxt_save(void)
 	asm volatile("movq %%cr2, %0" : "=a"(ctxt->cr2));
 	asm volatile("movq %%cr8, %0" : "=a"(ctxt->cr8));
 
-	asm volatile("movl %%cs, %%eax" : "=a"(ctxt->cs));
-	asm volatile("movl %%ss, %%eax" : "=a"(ctxt->ss));
-	asm volatile("movl %%ds, %%eax" : "=a"(ctxt->ds));
-	asm volatile("movl %%es, %%eax" : "=a"(ctxt->es));
-	asm volatile("movl %%fs, %%eax" : "=a"(ctxt->fs));
-	asm volatile("movl %%gs, %%eax" : "=a"(ctxt->gs));
+	asm volatile("movw %%cs, %0" : "=m"(ctxt->cs));
+	asm volatile("movw %%ss, %0" : "=m"(ctxt->ss));
+	asm volatile("movw %%ds, %0" : "=m"(ctxt->ds));
+	asm volatile("movw %%es, %0" : "=m"(ctxt->es));
+	asm volatile("movw %%fs, %0" : "=m"(ctxt->fs));
+	asm volatile("movw %%gs, %0" : "=m"(ctxt->gs));
 
 	native_store_gdt(&ctxt->gdtr);
 	store_idt(&ctxt->idtr);
-- 
2.53.0

[PATCH -hyperv 2/3] x86/hyperv: Use current_stack_pointer to avoid asm() in hv_hvcrash_ctxt_save()

[Uros Bizjak]

Use current_stack_pointer to avoid asm() when saving %rsp to the
crash context memory in hv_hvcrash_ctxt_save(). The new code is
more readable and results in exactly the same object file.

Signed-off-by: Uros Bizjak <ubizjak@gmail.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: Long Li <longli@microsoft.com>
Cc: Thomas Gleixner <tglx@kernel.org>
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/hyperv/hv_crash.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/hv_crash.c b/arch/x86/hyperv/hv_crash.c
index 2c7ea7e70854..d0f95a278fdb 100644
--- a/arch/x86/hyperv/hv_crash.c
+++ b/arch/x86/hyperv/hv_crash.c
@@ -199,7 +199,7 @@ static void hv_hvcrash_ctxt_save(void)
 {
 	struct hv_crash_ctxt *ctxt = &hv_crash_ctxt;
 
-	asm volatile("movq %%rsp,%0" : "=m"(ctxt->rsp));
+	ctxt->rsp = current_stack_pointer;
 
 	ctxt->cr0 = native_read_cr0();
 	ctxt->cr4 = native_read_cr4();
-- 
2.53.0

[PATCH -hyperv 3/3] x86/hyperv: Use any general-purpose register when saving %cr2 and %cr8

[Uros Bizjak]

hv_hvcrash_ctxt_save() in arch/x86/hyperv/hv_crash.c currently saves %cr2
and %cr8 using %eax ("=a"). This unnecessarily forces a specific register.
Update the inline assembly to use a general-purpose register ("=r") for
both %cr2 and %cr8. This makes the code more flexible for the compiler
while producing the same saved context contents.

No functional changes.

Signed-off-by: Uros Bizjak <ubizjak@gmail.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: Long Li <longli@microsoft.com>
Cc: Thomas Gleixner <tglx@kernel.org>
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/hyperv/hv_crash.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/hyperv/hv_crash.c b/arch/x86/hyperv/hv_crash.c
index d0f95a278fdb..5ffcc23255de 100644
--- a/arch/x86/hyperv/hv_crash.c
+++ b/arch/x86/hyperv/hv_crash.c
@@ -204,8 +204,8 @@ static void hv_hvcrash_ctxt_save(void)
 	ctxt->cr0 = native_read_cr0();
 	ctxt->cr4 = native_read_cr4();
 
-	asm volatile("movq %%cr2, %0" : "=a"(ctxt->cr2));
-	asm volatile("movq %%cr8, %0" : "=a"(ctxt->cr8));
+	asm volatile("movq %%cr2, %0" : "=r"(ctxt->cr2));
+	asm volatile("movq %%cr8, %0" : "=r"(ctxt->cr8));
 
 	asm volatile("movw %%cs, %0" : "=m"(ctxt->cs));
 	asm volatile("movw %%ss, %0" : "=m"(ctxt->ss));
-- 
2.53.0

Re: [PATCH -hyperv 1/3] x86/hyperv: Save segment registers directly to memory in hv_hvcrash_ctxt_save()

[Wei Liu]

On Wed, Mar 11, 2026 at 11:25:58AM +0100, Uros Bizjak wrote:
> hv_hvcrash_ctxt_save() in arch/x86/hyperv/hv_crash.c currently saves
> segment registers via a general-purpose register (%eax). Update the
> code to save segment registers (cs, ss, ds, es, fs, gs) directly to
> the crash context memory using movw. This avoids unnecessary use of
> a general-purpose register, making the code simpler and more efficient.
> 
> The size of the corresponding object file improves as follows:
> 
>    text    data     bss     dec     hex filename
>    4167     176     200    4543    11bf hv_crash-old.o
>    4151     176     200    4527    11af hv_crash-new.o
> 
> No functional change occurs to the saved context contents; this is
> purely a code-quality improvement.
> 
> Signed-off-by: Uros Bizjak <ubizjak@gmail.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: Long Li <longli@microsoft.com>
> Cc: Thomas Gleixner <tglx@kernel.org>
> 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>

Series applied to hyperv-fixes. Thanks.