[RFC PATCH 4/7] KVM: Refactor Assembly-code to access vCPU gp-registers through a macro

Fares Mehanna
2024-09-11T14:38:03Z

Right now assembly code accesses vCPU gp-regs directly from the context struct
"struct kvm_cpu_context" using "CPU_XREG_OFFSET()".

Since we want to move gp-regs to dynamic memory, we can no longer assume that
gp-regs will be embedded in the context struct, thus split the access to two
steps.

The first is to get the gp-regs from the context using the assembly macro
"get_ctxt_gp_regs".

And the second is to access the gp-registers directly from within the
"struct user_pt_regs" by removing the offset "CPU_USER_PT_REGS" from the access
macro "CPU_XREG_OFFSET()".

I also changed variable naming and comments where appropriate.

Signed-off-by: Fares Mehanna <faresx@amazon.de>
---
arch/arm64/include/asm/kvm_asm.h | 48 +++++++++++++++++---------------
arch/arm64/kvm/hyp/entry.S | 15 ++++++++++
arch/arm64/kvm/hyp/nvhe/host.S | 20 ++++++++++---
3 files changed, 57 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 2181a11b9d92..fa4fb642a5f5 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -313,6 +313,10 @@ void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr, u64 elr_virt,
str \vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
.endm

+.macro get_ctxt_gp_regs ctxt, regs
+ add \regs, \ctxt, #CPU_USER_PT_REGS
+.endm
+
/*
* KVM extable for unexpected exceptions.
* Create a struct kvm_exception_table_entry output to a section that can be
@@ -329,7 +333,7 @@ void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr, u64 elr_virt,
.popsection
.endm

-#define CPU_XREG_OFFSET(x) (CPU_USER_PT_REGS + 8*x)
+#define CPU_XREG_OFFSET(x) (8 * (x))
#define CPU_LR_OFFSET CPU_XREG_OFFSET(30)
#define CPU_SP_EL0_OFFSET (CPU_LR_OFFSET + 8)

@@ -337,34 +341,34 @@ void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr, u64 elr_virt,
* We treat x18 as callee-saved as the host may use it as a platform
* register (e.g. for shadow call stack).
*/
-.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)]
- stp x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
- stp x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
- stp x29, lr, [\ctxt, #CPU_XREG_OFFSET(29)]
+.macro save_callee_saved_regs regs
+ str x18, [\regs, #CPU_XREG_OFFSET(18)]
+ stp x19, x20, [\regs, #CPU_XREG_OFFSET(19)]
+ stp x21, x22, [\regs, #CPU_XREG_OFFSET(21)]
+ stp x23, x24, [\regs, #CPU_XREG_OFFSET(23)]
+ stp x25, x26, [\regs, #CPU_XREG_OFFSET(25)]
+ stp x27, x28, [\regs, #CPU_XREG_OFFSET(27)]
+ stp x29, lr, [\regs, #CPU_XREG_OFFSET(29)]
.endm

-.macro restore_callee_saved_regs ctxt
- // We require \ctxt is not x18-x28
- ldr x18, [\ctxt, #CPU_XREG_OFFSET(18)]
- ldp x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
- ldp x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
- ldp x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
- ldp x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
- ldp x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
- ldp x29, lr, [\ctxt, #CPU_XREG_OFFSET(29)]
+.macro restore_callee_saved_regs regs
+ // We require \regs is not x18-x28
+ ldr x18, [\regs, #CPU_XREG_OFFSET(18)]
+ ldp x19, x20, [\regs, #CPU_XREG_OFFSET(19)]
+ ldp x21, x22, [\regs, #CPU_XREG_OFFSET(21)]
+ ldp x23, x24, [\regs, #CPU_XREG_OFFSET(23)]
+ ldp x25, x26, [\regs, #CPU_XREG_OFFSET(25)]
+ ldp x27, x28, [\regs, #CPU_XREG_OFFSET(27)]
+ ldp x29, lr, [\regs, #CPU_XREG_OFFSET(29)]
.endm

-.macro save_sp_el0 ctxt, tmp
+.macro save_sp_el0 regs, tmp
mrs \tmp, sp_el0
- str \tmp, [\ctxt, #CPU_SP_EL0_OFFSET]
+ str \tmp, [\regs, #CPU_SP_EL0_OFFSET]
.endm

-.macro restore_sp_el0 ctxt, tmp
- ldr \tmp, [\ctxt, #CPU_SP_EL0_OFFSET]
+.macro restore_sp_el0 regs, tmp
+ ldr \tmp, [\regs, #CPU_SP_EL0_OFFSET]
msr sp_el0, \tmp
.endm

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index 4433a234aa9b..628a123bcdc1 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -28,6 +28,9 @@ SYM_FUNC_START(__guest_enter)

adr_this_cpu x1, kvm_hyp_ctxt, x2

+ // Get gp-regs pointer from the context
+ get_ctxt_gp_regs x1, x1
+
// Store the hyp regs
save_callee_saved_regs x1

@@ -62,6 +65,9 @@ alternative_else_nop_endif
// when this feature is enabled for kernel code.
ptrauth_switch_to_guest x29, x0, x1, x2

+ // Get gp-regs pointer from the context
+ get_ctxt_gp_regs x29, x29
+
// Restore the guest's sp_el0
restore_sp_el0 x29, x0

@@ -108,6 +114,7 @@ SYM_INNER_LABEL(__guest_exit_panic, SYM_L_GLOBAL)
// current state is saved to the guest context but it will only be
// accurate if the guest had been completely restored.
adr_this_cpu x0, kvm_hyp_ctxt, x1
+ get_ctxt_gp_regs x0, x0
adr_l x1, hyp_panic
str x1, [x0, #CPU_XREG_OFFSET(30)]

@@ -120,6 +127,7 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
// vcpu x0-x1 on the stack

add x1, x1, #VCPU_CONTEXT
+ get_ctxt_gp_regs x1, x1

ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN)

@@ -145,6 +153,10 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
// Store the guest's sp_el0
save_sp_el0 x1, x2

+ // Recover vCPU context to x1
+ get_vcpu_ptr x1, x2
+ add x1, x1, #VCPU_CONTEXT
+
adr_this_cpu x2, kvm_hyp_ctxt, x3

// Macro ptrauth_switch_to_hyp format:
@@ -157,6 +169,9 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
// mte_switch_to_hyp(g_ctxt, h_ctxt, reg1)
mte_switch_to_hyp x1, x2, x3

+ // Get gp-regs pointer from the context
+ get_ctxt_gp_regs x2, x2
+
// Restore hyp's sp_el0
restore_sp_el0 x2, x3

diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
index 3d610fc51f4d..31afa7396294 100644
--- a/arch/arm64/kvm/hyp/nvhe/host.S
+++ b/arch/arm64/kvm/hyp/nvhe/host.S
@@ -17,6 +17,12 @@
SYM_FUNC_START(__host_exit)
get_host_ctxt x0, x1

+ /* Keep host context in x1 */
+ mov x1, x0
+
+ /* Get gp-regs pointer from the context */
+ get_ctxt_gp_regs x0, x0
+
/* Store the host regs x2 and x3 */
stp x2, x3, [x0, #CPU_XREG_OFFSET(2)]

@@ -36,7 +42,10 @@ SYM_FUNC_START(__host_exit)
/* Store the host regs x18-x29, lr */
save_callee_saved_regs x0

- /* Save the host context pointer in x29 across the function call */
+ /* Save the host context pointer in x28 across the function call */
+ mov x28, x1
+
+ /* Save the host gp-regs pointer in x29 across the function call */
mov x29, x0

#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
@@ -46,7 +55,7 @@ alternative_else_nop_endif

alternative_if ARM64_KVM_PROTECTED_MODE
/* Save kernel ptrauth keys. */
- add x18, x29, #CPU_APIAKEYLO_EL1
+ add x18, x28, #CPU_APIAKEYLO_EL1
ptrauth_save_state x18, x19, x20

/* Use hyp keys. */
@@ -58,6 +67,7 @@ alternative_else_nop_endif
__skip_pauth_save:
#endif /* CONFIG_ARM64_PTR_AUTH_KERNEL */

+ mov x0, x28
bl handle_trap

__host_enter_restore_full:
@@ -68,7 +78,7 @@ b __skip_pauth_restore
alternative_else_nop_endif

alternative_if ARM64_KVM_PROTECTED_MODE
- add x18, x29, #CPU_APIAKEYLO_EL1
+ add x18, x28, #CPU_APIAKEYLO_EL1
ptrauth_restore_state x18, x19, x20
alternative_else_nop_endif
__skip_pauth_restore:
@@ -101,7 +111,8 @@ SYM_FUNC_END(__host_exit)
* void __noreturn __host_enter(struct kvm_cpu_context *host_ctxt);
*/
SYM_FUNC_START(__host_enter)
- mov x29, x0
+ mov x28, x0
+ get_ctxt_gp_regs x0, x29
b __host_enter_restore_full
SYM_FUNC_END(__host_enter)

@@ -141,6 +152,7 @@ SYM_FUNC_START(__hyp_do_panic)

/* Enter the host, conditionally restoring the host context. */
cbz x29, __host_enter_without_restoring
+ get_ctxt_gp_regs x29, x29
b __host_enter_for_panic
SYM_FUNC_END(__hyp_do_panic)

--
2.40.1