[PATCH 01/37] KVM: arm64: Avoid storing the vcpu pointer on the stack
Andrew Jones
drjones at redhat.com
Mon Nov 6 09:22:51 PST 2017
On Thu, Oct 12, 2017 at 12:41:05PM +0200, Christoffer Dall wrote:
> 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 requires us to have a scratch register though, so we take the
> chance to rearrange some of the el1_sync code to only look at the
> vttbr_el2 to determine if this is a trap from the guest or an HVC from
> the host. We do add an extra check to call the panic code if the kernel
> is configured with debugging enabled and we saw a trap from the host
> which wasn't an HVC, indicating that we left some EL2 trap configured by
> mistake.
>
> Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
> ---
> arch/arm64/include/asm/kvm_asm.h | 20 ++++++++++++++++++++
> arch/arm64/kernel/asm-offsets.c | 1 +
> arch/arm64/kvm/hyp/entry.S | 5 +----
> arch/arm64/kvm/hyp/hyp-entry.S | 39 ++++++++++++++++++---------------------
> arch/arm64/kvm/hyp/switch.c | 2 +-
> 5 files changed, 41 insertions(+), 26 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index ab4d0a9..7e48a39 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -70,4 +70,24 @@ extern u32 __init_stage2_translation(void);
>
> #endif
>
> +#ifdef __ASSEMBLY__
> +.macro get_host_ctxt reg, tmp
> + /*
> + * '=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 \reg, =kvm_host_cpu_state
> + mrs \tmp, tpidr_el2
> + add \reg, \reg, \tmp
> + kern_hyp_va \reg
> +.endm
> +
> +.macro get_vcpu vcpu, ctxt
> + ldr \vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
> + kern_hyp_va \vcpu
> +.endm
To avoid the need for the pattern
get_host_ctxt x0, x1
get_vcpu x1, x0
everywhere this macro is used, how about defining it as
.macro get_vcpu vcpu, tmp
get_host_ctxt \tmp, \vcpu
ldr \vcpu, [\tmp, #HOST_CONTEXT_VCPU]
kern_hyp_va \vcpu
.endm
which also has the side-effect of tmp being ctxt after the call.
> +
> +#endif
> +
> #endif /* __ARM_KVM_ASM_H__ */
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 71bf088..612021d 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -135,6 +135,7 @@ 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));
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index 9a8ab5d..76cd48f 100644
> --- 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
> @@ -119,7 +116,7 @@ ENTRY(__guest_exit)
> 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
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> index e4f37b9..2950f26 100644
> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -56,19 +56,16 @@ 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
> -
> - cmp x0, #ESR_ELx_EC_HVC64
> - b.ne el1_trap
> -
> mrs x1, vttbr_el2 // If vttbr is valid, the 64bit guest
> cbnz x1, el1_trap // called HVC
>
> +#ifdef CONFIG_DEBUG
> + mrs x0, esr_el2
> + lsr x0, x0, #ESR_ELx_EC_SHIFT
> + cmp x0, #ESR_ELx_EC_HVC64
> + b.ne __hyp_panic
> +#endif
The dropping of the "alternative_if_not ARM64_HAS_VIRT_HOST_EXTN" stuff
isn't called out in the commit message, but it looks like it was just
a cleanup of code that was never necessary, as esr_el2 aliases esr_el1.
Is that correct?
> +
> /* Here, we're pretty sure the host called HVC. */
> ldp x0, x1, [sp], #16
>
> @@ -101,10 +98,15 @@ alternative_endif
> eret
>
> el1_trap:
> + get_host_ctxt x0, x1
> + get_vcpu 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
> @@ -122,13 +124,15 @@ alternative_else_nop_endif
>
> el1_irq:
> stp x0, x1, [sp, #-16]!
> - ldr x1, [sp, #16 + 8]
> + get_host_ctxt x0, x1
> + get_vcpu x1, x0
> mov x0, #ARM_EXCEPTION_IRQ
> b __guest_exit
>
> el1_error:
> stp x0, x1, [sp, #-16]!
> - ldr x1, [sp, #16 + 8]
> + get_host_ctxt x0, x1
> + get_vcpu x1, x0
> mov x0, #ARM_EXCEPTION_EL1_SERROR
> b __guest_exit
>
> @@ -164,14 +168,7 @@ 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)
>
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 69ef24a..a0123ad 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -435,7 +435,7 @@ void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *__host_ctxt)
> if (read_sysreg(vttbr_el2)) {
> struct kvm_cpu_context *host_ctxt;
>
> - host_ctxt = kern_hyp_va(__host_ctxt);
> + host_ctxt = __host_ctxt;
> vcpu = host_ctxt->__hyp_running_vcpu;
> __timer_disable_traps(vcpu);
> __deactivate_traps(vcpu);
> --
> 2.9.0
>
Thanks,
drew
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