[PATCH v2 0/3] x86/xen: some xen_hypercall_hvm() fixes

[PATCH v2 0/3] x86/xen: some xen_hypercall_hvm() fixes

[Juergen Gross]

3 fixes of the xen_hypercall_hvm() function, with the last one being
probably more a cleanup.

Juergen Gross (3):
  x86/xen: fix xen_hypercall_hvm() to not clobber %rbx
  x86/xen: add FRAME_END to xen_hypercall_hvm()
  x86/xen: remove unneeded dummy push from xen_hypercall_hvm()

 arch/x86/xen/xen-head.S | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

-- 
2.43.0

[PATCH v2 1/3] x86/xen: fix xen_hypercall_hvm() to not clobber %rbx

[Juergen Gross]

xen_hypercall_hvm(), which is used when running as a Xen PVH guest at
most only once during early boot, is clobbering %rbx. Depending on
whether the caller relies on %rbx to be preserved across the call or
not, this clobbering might result in an early crash of the system.

This can be avoided by using an already saved register instead of %rbx.

Fixes: b4845bb63838 ("x86/xen: add central hypercall functions")
Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- use %rcx instead of %rbx, keeping the lea instruction (Jan Beulich)
---
 arch/x86/xen/xen-head.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 9252652afe59..72f28d66e0e5 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -117,8 +117,8 @@ SYM_FUNC_START(xen_hypercall_hvm)
 	pop %ebx
 	pop %eax
 #else
-	lea xen_hypercall_amd(%rip), %rbx
-	cmp %rax, %rbx
+	lea xen_hypercall_amd(%rip), %rcx
+	cmp %rax, %rcx
 #ifdef CONFIG_FRAME_POINTER
 	pop %rax	/* Dummy pop. */
 #endif
-- 
2.43.0

[PATCH v2 2/3] x86/xen: add FRAME_END to xen_hypercall_hvm()

[Juergen Gross]

xen_hypercall_hvm() is missing a FRAME_END at the end, add it.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502030848.HTNTTuo9-lkp@intel.com/
Fixes: b4845bb63838 ("x86/xen: add central hypercall functions")
Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/xen/xen-head.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 72f28d66e0e5..4e481b0eefc9 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -132,6 +132,7 @@ SYM_FUNC_START(xen_hypercall_hvm)
 	pop %rcx
 	pop %rax
 #endif
+	FRAME_END
 	/* Use correct hypercall function. */
 	jz xen_hypercall_amd
 	jmp xen_hypercall_intel
-- 
2.43.0

[PATCH v2 3/3] x86/xen: remove unneeded dummy push from xen_hypercall_hvm()

[Juergen Gross]

Stack alignment of the kernel in 64-bit mode is 8, not 16, so the
dummy push in xen_hypercall_hvm() for aligning the stack to 16 bytes
can be removed.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/xen/xen-head.S | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 4e481b0eefc9..894edf8d6d62 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -100,9 +100,6 @@ SYM_FUNC_START(xen_hypercall_hvm)
 	push %r10
 	push %r9
 	push %r8
-#ifdef CONFIG_FRAME_POINTER
-	pushq $0	/* Dummy push for stack alignment. */
-#endif
 #endif
 	/* Set the vendor specific function. */
 	call __xen_hypercall_setfunc
@@ -119,9 +116,6 @@ SYM_FUNC_START(xen_hypercall_hvm)
 #else
 	lea xen_hypercall_amd(%rip), %rcx
 	cmp %rax, %rcx
-#ifdef CONFIG_FRAME_POINTER
-	pop %rax	/* Dummy pop. */
-#endif
 	pop %r8
 	pop %r9
 	pop %r10
-- 
2.43.0

Re: [PATCH v2 0/3] x86/xen: some xen_hypercall_hvm() fixes

[Jan Beulich]

On 05.02.2025 12:06, Juergen Gross wrote:
> 3 fixes of the xen_hypercall_hvm() function, with the last one being
> probably more a cleanup.
> 
> Juergen Gross (3):
>   x86/xen: fix xen_hypercall_hvm() to not clobber %rbx
>   x86/xen: add FRAME_END to xen_hypercall_hvm()
>   x86/xen: remove unneeded dummy push from xen_hypercall_hvm()

Reviewed-by: Jan Beulich <jbeulich@suse.com>

Re: [PATCH v2 0/3] x86/xen: some xen_hypercall_hvm() fixes

[Andrew Cooper]

On 05/02/2025 11:06 am, Juergen Gross wrote:
> 3 fixes of the xen_hypercall_hvm() function, with the last one being
> probably more a cleanup.
>
> Juergen Gross (3):
>   x86/xen: fix xen_hypercall_hvm() to not clobber %rbx
>   x86/xen: add FRAME_END to xen_hypercall_hvm()
>   x86/xen: remove unneeded dummy push from xen_hypercall_hvm()
>
>  arch/x86/xen/xen-head.S | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>