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Subject[PATCH 4.14 77/92] KVM: arm64: Avoid storing the vcpu pointer on the stack
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christoffer Dall <christoffer.dall@linaro.org>

Commit 4464e210de9e80e38de59df052fe09ea2ff80b1b upstream.

We already have the percpu area for the host cpu state, which points to
the VCPU, so there's no need to store the VCPU pointer on the stack on
every context switch. We can be a little more clever and just use
tpidr_el2 for the percpu offset and load the VCPU pointer from the host
context.

This has the benefit of being able to retrieve the host context even
when our stack is corrupted, and it has a potential performance benefit
because we trade a store plus a load for an mrs and a load on a round
trip to the guest.

This does require us to calculate the percpu offset without including
the offset from the kernel mapping of the percpu array to the linear
mapping of the array (which is what we store in tpidr_el1), because a
PC-relative generated address in EL2 is already giving us the hyp alias
of the linear mapping of a kernel address. We do this in
__cpu_init_hyp_mode() by using kvm_ksym_ref().

The code that accesses ESR_EL2 was previously using an alternative to
use the _EL1 accessor on VHE systems, but this was actually unnecessary
as the _EL1 accessor aliases the ESR_EL2 register on VHE, and the _EL2
accessor does the same thing on both systems.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/include/asm/kvm_asm.h | 15 +++++++++++++++
arch/arm64/include/asm/kvm_host.h | 15 +++++++++++++++
arch/arm64/kernel/asm-offsets.c | 1 +
arch/arm64/kvm/hyp/entry.S | 6 +-----
arch/arm64/kvm/hyp/hyp-entry.S | 28 ++++++++++------------------
arch/arm64/kvm/hyp/switch.c | 5 +----
arch/arm64/kvm/hyp/sysreg-sr.c | 5 +++++
7 files changed, 48 insertions(+), 27 deletions(-)

--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -33,6 +33,7 @@
#define KVM_ARM64_DEBUG_DIRTY_SHIFT 0
#define KVM_ARM64_DEBUG_DIRTY (1 << KVM_ARM64_DEBUG_DIRTY_SHIFT)

+/* Translate a kernel address of @sym into its equivalent linear mapping */
#define kvm_ksym_ref(sym) \
({ \
void *val = &sym; \
@@ -68,6 +69,20 @@ extern u32 __init_stage2_translation(voi

extern void __qcom_hyp_sanitize_btac_predictors(void);

+#else /* __ASSEMBLY__ */
+
+.macro get_host_ctxt reg, tmp
+ adr_l \reg, kvm_host_cpu_state
+ mrs \tmp, tpidr_el2
+ add \reg, \reg, \tmp
+.endm
+
+.macro get_vcpu_ptr vcpu, ctxt
+ get_host_ctxt \ctxt, \vcpu
+ ldr \vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
+ kern_hyp_va \vcpu
+.endm
+
#endif

#endif /* __ARM_KVM_ASM_H__ */
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -350,10 +350,15 @@ int kvm_perf_teardown(void);

struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);

+void __kvm_set_tpidr_el2(u64 tpidr_el2);
+DECLARE_PER_CPU(kvm_cpu_context_t, kvm_host_cpu_state);
+
static inline void __cpu_init_hyp_mode(phys_addr_t pgd_ptr,
unsigned long hyp_stack_ptr,
unsigned long vector_ptr)
{
+ u64 tpidr_el2;
+
/*
* Call initialization code, and switch to the full blown HYP code.
* If the cpucaps haven't been finalized yet, something has gone very
@@ -362,6 +367,16 @@ static inline void __cpu_init_hyp_mode(p
*/
BUG_ON(!static_branch_likely(&arm64_const_caps_ready));
__kvm_call_hyp((void *)pgd_ptr, hyp_stack_ptr, vector_ptr);
+
+ /*
+ * Calculate the raw per-cpu offset without a translation from the
+ * kernel's mapping to the linear mapping, and store it in tpidr_el2
+ * so that we can use adr_l to access per-cpu variables in EL2.
+ */
+ tpidr_el2 = (u64)this_cpu_ptr(&kvm_host_cpu_state)
+ - (u64)kvm_ksym_ref(kvm_host_cpu_state);
+
+ kvm_call_hyp(__kvm_set_tpidr_el2, tpidr_el2);
}

static inline void kvm_arch_hardware_unsetup(void) {}
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -136,6 +136,7 @@ int main(void)
DEFINE(CPU_FP_REGS, offsetof(struct kvm_regs, fp_regs));
DEFINE(VCPU_FPEXC32_EL2, offsetof(struct kvm_vcpu, arch.ctxt.sys_regs[FPEXC32_EL2]));
DEFINE(VCPU_HOST_CONTEXT, offsetof(struct kvm_vcpu, arch.host_cpu_context));
+ DEFINE(HOST_CONTEXT_VCPU, offsetof(struct kvm_cpu_context, __hyp_running_vcpu));
#endif
#ifdef CONFIG_CPU_PM
DEFINE(CPU_SUSPEND_SZ, sizeof(struct cpu_suspend_ctx));
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -62,9 +62,6 @@ ENTRY(__guest_enter)
// Store the host regs
save_callee_saved_regs x1

- // Store host_ctxt and vcpu for use at exit time
- stp x1, x0, [sp, #-16]!
-
add x18, x0, #VCPU_CONTEXT

// Restore guest regs x0-x17
@@ -118,8 +115,7 @@ ENTRY(__guest_exit)
// Store the guest regs x19-x29, lr
save_callee_saved_regs x1

- // Restore the host_ctxt from the stack
- ldr x2, [sp], #16
+ get_host_ctxt x2, x3

// Now restore the host regs
restore_callee_saved_regs x2
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -57,13 +57,8 @@ ENDPROC(__vhe_hyp_call)
el1_sync: // Guest trapped into EL2
stp x0, x1, [sp, #-16]!

-alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
- mrs x1, esr_el2
-alternative_else
- mrs x1, esr_el1
-alternative_endif
- lsr x0, x1, #ESR_ELx_EC_SHIFT
-
+ mrs x0, esr_el2
+ lsr x0, x0, #ESR_ELx_EC_SHIFT
cmp x0, #ESR_ELx_EC_HVC64
ccmp x0, #ESR_ELx_EC_HVC32, #4, ne
b.ne el1_trap
@@ -117,10 +112,14 @@ el1_hvc_guest:
eret

el1_trap:
+ get_vcpu_ptr x1, x0
+
+ mrs x0, esr_el2
+ lsr x0, x0, #ESR_ELx_EC_SHIFT
/*
* x0: ESR_EC
+ * x1: vcpu pointer
*/
- ldr x1, [sp, #16 + 8] // vcpu stored by __guest_enter

/*
* We trap the first access to the FP/SIMD to save the host context
@@ -138,13 +137,13 @@ alternative_else_nop_endif

el1_irq:
stp x0, x1, [sp, #-16]!
- ldr x1, [sp, #16 + 8]
+ get_vcpu_ptr x1, x0
mov x0, #ARM_EXCEPTION_IRQ
b __guest_exit

el1_error:
stp x0, x1, [sp, #-16]!
- ldr x1, [sp, #16 + 8]
+ get_vcpu_ptr x1, x0
mov x0, #ARM_EXCEPTION_EL1_SERROR
b __guest_exit

@@ -180,14 +179,7 @@ ENTRY(__hyp_do_panic)
ENDPROC(__hyp_do_panic)

ENTRY(__hyp_panic)
- /*
- * '=kvm_host_cpu_state' is a host VA from the constant pool, it may
- * not be accessible by this address from EL2, hyp_panic() converts
- * it with kern_hyp_va() before use.
- */
- ldr x0, =kvm_host_cpu_state
- mrs x1, tpidr_el2
- add x0, x0, x1
+ get_host_ctxt x0, x1
b hyp_panic
ENDPROC(__hyp_panic)

--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -437,7 +437,7 @@ static hyp_alternate_select(__hyp_call_p
__hyp_call_panic_nvhe, __hyp_call_panic_vhe,
ARM64_HAS_VIRT_HOST_EXTN);

-void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *__host_ctxt)
+void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *host_ctxt)
{
struct kvm_vcpu *vcpu = NULL;

@@ -446,9 +446,6 @@ void __hyp_text __noreturn hyp_panic(str
u64 par = read_sysreg(par_el1);

if (read_sysreg(vttbr_el2)) {
- struct kvm_cpu_context *host_ctxt;
-
- host_ctxt = kern_hyp_va(__host_ctxt);
vcpu = host_ctxt->__hyp_running_vcpu;
__timer_save_state(vcpu);
__deactivate_traps(vcpu);
--- a/arch/arm64/kvm/hyp/sysreg-sr.c
+++ b/arch/arm64/kvm/hyp/sysreg-sr.c
@@ -183,3 +183,8 @@ void __hyp_text __sysreg32_restore_state
if (vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
write_sysreg(sysreg[DBGVCR32_EL2], dbgvcr32_el2);
}
+
+void __hyp_text __kvm_set_tpidr_el2(u64 tpidr_el2)
+{
+ asm("msr tpidr_el2, %0": : "r" (tpidr_el2));
+}

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