Subject: [PATCH 01/37] KVM: arm64: Avoid storing the vcpu pointer on the stack

From: [email-masked] (Christoffer Dall)
Date: Tue, 7 Nov 2017 09:24:08 +0100

On Mon, Nov 06, 2017 at 06:22:51PM +0100, Andrew Jones wrote:
> 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 <[email-masked]>
> > ---
> >  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.
> 

I actually prefer it the way it is now, because I think it's clearer
what's going on.  In the past we did some information hiding in assembly
macros and that didn't improve maintenance.

If there's an aspect to this I'm missing, I'm of course open to
reconsiderig though.

> > +
> > +#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?
> 

I suppose 'rearranging some of the code' is not a very precise
description.  With VHE, using the 'mrs x1, esr_el1' instruction, will
actuall read ESR_EL2 into x1, and using the 'mrs x1, esr_el2'
instruction will have the same effect on VHE, so we might as well simply
use that.

I'll adjust the commit message.

Thanks for looking at this!
-Christoffer

> > +
> >  	/* 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
> >