[PATCH v4 13/46] KVM: PPC: Book3S 64: Move GUEST_MODE_SKIP test into KVM

[Paul Mackerras]

On Tue, Mar 23, 2021 at 11:02:32AM +1000, Nicholas Piggin wrote:
> Move the GUEST_MODE_SKIP logic into KVM code. This is quite a KVM
> internal detail that has no real need to be in common handlers.
> 
> Also add a comment explaining why this thing exists.

[snip]

> diff --git a/arch/powerpc/kvm/book3s_64_entry.S b/arch/powerpc/kvm/book3s_64_entry.S
> index 7a039ea78f15..a5412e24cc05 100644
> --- a/arch/powerpc/kvm/book3s_64_entry.S
> +++ b/arch/powerpc/kvm/book3s_64_entry.S
> @@ -1,6 +1,7 @@
>  /* SPDX-License-Identifier: GPL-2.0-only */
>  #include <asm/asm-offsets.h>
>  #include <asm/cache.h>
> +#include <asm/exception-64s.h>
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_book3s_asm.h>
>  #include <asm/ppc_asm.h>
> @@ -20,9 +21,12 @@ kvmppc_interrupt:
>  	 * guest R12 saved in shadow VCPU SCRATCH0
>  	 * guest R13 saved in SPRN_SCRATCH0
>  	 */
> -#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
>  	std	r9,HSTATE_SCRATCH2(r13)
>  	lbz	r9,HSTATE_IN_GUEST(r13)
> +	cmpwi	r9,KVM_GUEST_MODE_SKIP
> +	beq-	.Lmaybe_skip
> +.Lno_skip:
> +#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
>  	cmpwi	r9,KVM_GUEST_MODE_HOST_HV
>  	beq	kvmppc_bad_host_intr
>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
> @@ -34,3 +38,48 @@ kvmppc_interrupt:
>  #else
>  	b	kvmppc_interrupt_pr
>  #endif

It's a bit hard to see without more context, but I think that in the
PR-only case (CONFIG_KVM_BOOK3S_HV_POSSIBLE undefined), this will
corrupt R9.  You need to restore R9 before the unconditional branch to
kvmppc_interrupt_pr.  (I realize this code gets modified further, but
I'd rather not break bisection.)

Paul.