diff mbox series

[03/18] arm64: kvm: stop treating register x18 as caller save

Message ID 20191018161033.261971-4-samitolvanen@google.com (mailing list archive)
State New, archived
Headers show
Series add support for Clang's Shadow Call Stack | expand

Commit Message

Sami Tolvanen Oct. 18, 2019, 4:10 p.m. UTC 
From: Ard Biesheuvel <ard.biesheuvel@linaro.org>

In preparation of using x18 as a task struct pointer register when
running in the kernel, stop treating it as caller save in the KVM
guest entry/exit code. Currently, the code assumes there is no need
to preserve it for the host, given that it would have been assumed
clobbered anyway by the function call to __guest_enter(). Instead,
preserve its value and restore it upon return.

Link: https://patchwork.kernel.org/patch/9836891/
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 arch/arm64/kvm/hyp/entry.S | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

Comments

Ard Biesheuvel Oct. 21, 2019, 6:19 a.m. UTC | #1 
On Fri, 18 Oct 2019 at 18:10, Sami Tolvanen <samitolvanen@google.com> wrote:
>
> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
> In preparation of using x18 as a task struct pointer register when
> running in the kernel, stop treating it as caller save in the KVM
> guest entry/exit code. Currently, the code assumes there is no need
> to preserve it for the host, given that it would have been assumed
> clobbered anyway by the function call to __guest_enter(). Instead,
> preserve its value and restore it upon return.
>
> Link: https://patchwork.kernel.org/patch/9836891/
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

You might want to update the commit log to drop the reference to the
task struct pointer.

> ---
>  arch/arm64/kvm/hyp/entry.S | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index e5cc8d66bf53..20bd9a20ea27 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -23,6 +23,7 @@
>         .pushsection    .hyp.text, "ax"
>
>  .macro save_callee_saved_regs ctxt
> +       str     x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
>         stp     x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
>         stp     x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
>         stp     x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
> @@ -38,6 +39,7 @@
>         ldp     x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
>         ldp     x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
>         ldp     x29, lr,  [\ctxt, #CPU_XREG_OFFSET(29)]
> +       ldr     x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
>  .endm
>
>  /*
> @@ -87,12 +89,9 @@ alternative_else_nop_endif
>         ldp     x14, x15, [x18, #CPU_XREG_OFFSET(14)]
>         ldp     x16, x17, [x18, #CPU_XREG_OFFSET(16)]
>
> -       // Restore guest regs x19-x29, lr
> +       // Restore guest regs x18-x29, lr
>         restore_callee_saved_regs x18
>
> -       // Restore guest reg x18
> -       ldr     x18,      [x18, #CPU_XREG_OFFSET(18)]
> -
>         // Do not touch any register after this!
>         eret
>         sb
> @@ -114,7 +113,7 @@ ENTRY(__guest_exit)
>         // Retrieve the guest regs x0-x1 from the stack
>         ldp     x2, x3, [sp], #16       // x0, x1
>
> -       // Store the guest regs x0-x1 and x4-x18
> +       // Store the guest regs x0-x1 and x4-x17
>         stp     x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
>         stp     x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
>         stp     x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
> @@ -123,9 +122,8 @@ ENTRY(__guest_exit)
>         stp     x12, x13, [x1, #CPU_XREG_OFFSET(12)]
>         stp     x14, x15, [x1, #CPU_XREG_OFFSET(14)]
>         stp     x16, x17, [x1, #CPU_XREG_OFFSET(16)]
> -       str     x18,      [x1, #CPU_XREG_OFFSET(18)]
>
> -       // Store the guest regs x19-x29, lr
> +       // Store the guest regs x18-x29, lr
>         save_callee_saved_regs x1
>
>         get_host_ctxt   x2, x3
> --
> 2.23.0.866.gb869b98d4c-goog
>
Marc Zyngier Oct. 22, 2019, 5:22 p.m. UTC | #2 
On Fri, 18 Oct 2019 09:10:18 -0700
Sami Tolvanen <samitolvanen@google.com> wrote:

> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> 
> In preparation of using x18 as a task struct pointer register when
> running in the kernel, stop treating it as caller save in the KVM
> guest entry/exit code. Currently, the code assumes there is no need
> to preserve it for the host, given that it would have been assumed
> clobbered anyway by the function call to __guest_enter(). Instead,
> preserve its value and restore it upon return.
> 
> Link: https://patchwork.kernel.org/patch/9836891/
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
>  arch/arm64/kvm/hyp/entry.S | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index e5cc8d66bf53..20bd9a20ea27 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -23,6 +23,7 @@
>  	.pushsection	.hyp.text, "ax"
>  
>  .macro save_callee_saved_regs ctxt
> +	str	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
>  	stp	x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
>  	stp	x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
>  	stp	x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
> @@ -38,6 +39,7 @@
>  	ldp	x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
>  	ldp	x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
>  	ldp	x29, lr,  [\ctxt, #CPU_XREG_OFFSET(29)]
> +	ldr	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]

There is now an assumption that ctxt is x18 (otherwise why would it be
out of order?). Please add a comment to that effect.

>  .endm
>  
>  /*
> @@ -87,12 +89,9 @@ alternative_else_nop_endif
>  	ldp	x14, x15, [x18, #CPU_XREG_OFFSET(14)]
>  	ldp	x16, x17, [x18, #CPU_XREG_OFFSET(16)]
>  
> -	// Restore guest regs x19-x29, lr
> +	// Restore guest regs x18-x29, lr
>  	restore_callee_saved_regs x18

Or you could elect another register such as x29 as the base, and keep
the above in a reasonable order.

>  
> -	// Restore guest reg x18
> -	ldr	x18,      [x18, #CPU_XREG_OFFSET(18)]
> -
>  	// Do not touch any register after this!
>  	eret
>  	sb
> @@ -114,7 +113,7 @@ ENTRY(__guest_exit)
>  	// Retrieve the guest regs x0-x1 from the stack
>  	ldp	x2, x3, [sp], #16	// x0, x1
>  
> -	// Store the guest regs x0-x1 and x4-x18
> +	// Store the guest regs x0-x1 and x4-x17
>  	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
>  	stp	x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
>  	stp	x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
> @@ -123,9 +122,8 @@ ENTRY(__guest_exit)
>  	stp	x12, x13, [x1, #CPU_XREG_OFFSET(12)]
>  	stp	x14, x15, [x1, #CPU_XREG_OFFSET(14)]
>  	stp	x16, x17, [x1, #CPU_XREG_OFFSET(16)]
> -	str	x18,      [x1, #CPU_XREG_OFFSET(18)]
>  
> -	// Store the guest regs x19-x29, lr
> +	// Store the guest regs x18-x29, lr
>  	save_callee_saved_regs x1
>  
>  	get_host_ctxt	x2, x3

Thanks,

	M.
Sami Tolvanen Oct. 22, 2019, 9:45 p.m. UTC | #3 
On Tue, Oct 22, 2019 at 10:22 AM Marc Zyngier <maz@kernel.org> wrote:
> >  .macro save_callee_saved_regs ctxt
> > +     str     x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
> >       stp     x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
> >       stp     x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
> >       stp     x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
> > @@ -38,6 +39,7 @@
> >       ldp     x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
> >       ldp     x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
> >       ldp     x29, lr,  [\ctxt, #CPU_XREG_OFFSET(29)]
> > +     ldr     x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
>
> There is now an assumption that ctxt is x18 (otherwise why would it be
> out of order?). Please add a comment to that effect.

> > -     // Restore guest regs x19-x29, lr
> > +     // Restore guest regs x18-x29, lr
> >       restore_callee_saved_regs x18
>
> Or you could elect another register such as x29 as the base, and keep
> the above in a reasonable order.

I'm fine with either option. Ard, any thoughts?

Sami
diff mbox series

Patch

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index e5cc8d66bf53..20bd9a20ea27 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -23,6 +23,7 @@ 
 	.pushsection	.hyp.text, "ax"
 
 .macro save_callee_saved_regs ctxt
+	str	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
 	stp	x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
 	stp	x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
 	stp	x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
@@ -38,6 +39,7 @@ 
 	ldp	x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
 	ldp	x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
 	ldp	x29, lr,  [\ctxt, #CPU_XREG_OFFSET(29)]
+	ldr	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
 .endm
 
 /*
@@ -87,12 +89,9 @@  alternative_else_nop_endif
 	ldp	x14, x15, [x18, #CPU_XREG_OFFSET(14)]
 	ldp	x16, x17, [x18, #CPU_XREG_OFFSET(16)]
 
-	// Restore guest regs x19-x29, lr
+	// Restore guest regs x18-x29, lr
 	restore_callee_saved_regs x18
 
-	// Restore guest reg x18
-	ldr	x18,      [x18, #CPU_XREG_OFFSET(18)]
-
 	// Do not touch any register after this!
 	eret
 	sb
@@ -114,7 +113,7 @@  ENTRY(__guest_exit)
 	// Retrieve the guest regs x0-x1 from the stack
 	ldp	x2, x3, [sp], #16	// x0, x1
 
-	// Store the guest regs x0-x1 and x4-x18
+	// Store the guest regs x0-x1 and x4-x17
 	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
 	stp	x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
 	stp	x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
@@ -123,9 +122,8 @@  ENTRY(__guest_exit)
 	stp	x12, x13, [x1, #CPU_XREG_OFFSET(12)]
 	stp	x14, x15, [x1, #CPU_XREG_OFFSET(14)]
 	stp	x16, x17, [x1, #CPU_XREG_OFFSET(16)]
-	str	x18,      [x1, #CPU_XREG_OFFSET(18)]
 
-	// Store the guest regs x19-x29, lr
+	// Store the guest regs x18-x29, lr
 	save_callee_saved_regs x1
 
 	get_host_ctxt	x2, x3