Re: [PATCH 3/3] KVM: PPC: Book3S: 64-bit CONFIG_RELOCATABLE support for interrupts

[Paul Mackerras <paulus@ozlabs.org>]

On Thu, Dec 22, 2016 at 04:29:27AM +1000, Nicholas Piggin wrote:
> 64-bit Book3S exception handlers must find the dynamic kernel base
> to add to the target address when branching beyond __end_interrupts,
> in order to support kernel running at non-0 physical address.
> 
> Support this in KVM by branching with CTR, similarly to regular
> interrupt handlers. The guest CTR saved in HSTATE_SCRATCH1 and
> restored after the branch.
> 
> Without this, the host kernel hangs and crashes randomly when it is
> running at a non-0 address and a KVM guest is started.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Unfortunately, this patch causes guests to crash in various ways when
running under a CONFIG_RELOCATABLE kernel.  See below...

> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index 11882aac8216..c18ce740452b 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -1060,15 +1060,16 @@ kvmppc_interrupt_hv:
>  	 * R12		= (guest CR << 32) | interrupt vector
>  	 * R13		= PACA
>  	 * guest R12 saved in shadow VCPU SCRATCH0
> +	 * guest CTR saved in shadow VCPU SCRATCH1 if RELOCATABLE
>  	 * guest R13 saved in SPRN_SCRATCH0
>  	 */
> -	std	r9, HSTATE_SCRATCH1(r13)
> +	std	r9, HSTATE_SCRATCH2(r13)
>  	lbz	r9, HSTATE_IN_GUEST(r13)
>  	cmpwi	r9, KVM_GUEST_MODE_HOST_HV
>  	beq	kvmppc_bad_host_intr
>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
>  	cmpwi	r9, KVM_GUEST_MODE_GUEST
> -	ld	r9, HSTATE_SCRATCH1(r13)
> +	ld	r9, HSTATE_SCRATCH2(r13)
>  	beq	kvmppc_interrupt_pr
>  #endif
>  	/* We're now back in the host but in guest MMU context */
> @@ -1088,7 +1089,7 @@ kvmppc_interrupt_hv:
>  	std	r6, VCPU_GPR(R6)(r9)
>  	std	r7, VCPU_GPR(R7)(r9)
>  	std	r8, VCPU_GPR(R8)(r9)
> -	ld	r0, HSTATE_SCRATCH1(r13)
> +	ld	r0, HSTATE_SCRATCH2(r13)
>  	std	r0, VCPU_GPR(R9)(r9)
>  	std	r10, VCPU_GPR(R10)(r9)
>  	std	r11, VCPU_GPR(R11)(r9)
> @@ -1151,7 +1152,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  11:	stw	r3,VCPU_HEIR(r9)
>  
>  	/* these are volatile across C function calls */
> +#ifdef CONFIG_RELOCATABLE
> +	ld	r3, HSTATE_SCRATCH1(r13)

The problem is here.  Depending on what sort of interrupt it is, we
can end up going back to the guest via fast_guest_return, which
doesn't reload CTR.  Adding

	mtctr	r3

at this point makes things work.

Paul.