Re: [PATCH v2] KVM: PPC: Book3S HV: Fix L2 guest reboot failure due to empty 'arch_compat'

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> Currently, rebooting a pseries nested qemu-kvm guest (L2) results in
> below error as L1 qemu sends PVR value 'arch_compat' == 0 via
> ppc_set_compat ioctl. This triggers a condition failure in
> kvmppc_set_arch_compat() resulting in an EINVAL.
>
> qemu-system-ppc64: Unable to set CPU compatibility mode in KVM: Invalid
> argument
>
> Also, a value of 0 for arch_compat generally refers the default
> compatibility of the host. But, arch_compat, being a Guest Wide Element
> in nested API v2, cannot be set to 0 in GSB as PowerVM (L0) expects a
> non-zero value. A value of 0 triggers a kernel trap during a reboot and
> consequently causes it to fail:
>
> [   22.106360] reboot: Restarting system
> KVM: unknown exit, hardware reason ffffffffffffffea
> NIP 0000000000000100   LR 000000000000fe44 CTR 0000000000000000 XER 0000000020040092 CPU#0
> MSR 0000000000001000 HID0 0000000000000000  HF 6c000000 iidx 3 didx 3
> TB 00000000 00000000 DECR 0
> GPR00 0000000000000000 0000000000000000 c000000002a8c300 000000007fe00000
> GPR04 0000000000000000 0000000000000000 0000000000001002 8000000002803033
> GPR08 000000000a000000 0000000000000000 0000000000000004 000000002fff0000
> GPR12 0000000000000000 c000000002e10000 0000000105639200 0000000000000004
> GPR16 0000000000000000 000000010563a090 0000000000000000 0000000000000000
> GPR20 0000000105639e20 00000001056399c8 00007fffe54abab0 0000000105639288
> GPR24 0000000000000000 0000000000000001 0000000000000001 0000000000000000
> GPR28 0000000000000000 0000000000000000 c000000002b30840 0000000000000000
> CR 00000000  [ -  -  -  -  -  -  -  -  ]     RES 000@ffffffffffffffff
>  SRR0 0000000000000000  SRR1 0000000000000000    PVR 0000000000800200 VRSAVE 0000000000000000
> SPRG0 0000000000000000 SPRG1 0000000000000000  SPRG2 0000000000000000  SPRG3 0000000000000000
> SPRG4 0000000000000000 SPRG5 0000000000000000  SPRG6 0000000000000000  SPRG7 0000000000000000
> HSRR0 0000000000000000 HSRR1 0000000000000000
>  CFAR 0000000000000000
>  LPCR 0000000000020400
>  PTCR 0000000000000000   DAR 0000000000000000  DSISR 0000000000000000
>
>  kernel:trap=0xffffffea | pc=0x100 | msr=0x1000
>
> This patch updates kvmppc_set_arch_compat() to use the host PVR value if
> 'compat_pvr' == 0 indicating that qemu doesn't want to enforce any
> specific PVR compat mode.
>

Reviewed-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>

>
> Fixes: 19d31c5f1157 ("KVM: PPC: Add support for nestedv2 guests")
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>
> Changes v1 -> v2:
>     - Added descriptive error log in the patch description when
>       `arch_compat == 0` passed in GSB
>     - Added a helper function for PCR to capabilities mapping
>     - Added relevant comments around the changes being made
>
> v1: https://lore.kernel.org/lkml/20240118095653.2588129-1-amachhiw@linux.ibm.com/
>
>  arch/powerpc/kvm/book3s_hv.c          | 25 +++++++++++++++++++++++--
>  arch/powerpc/kvm/book3s_hv_nestedv2.c | 23 +++++++++++++++++++++--
>  2 files changed, 44 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 52427fc2a33f..270ab9cf9a54 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -391,6 +391,23 @@ static void kvmppc_set_pvr_hv(struct kvm_vcpu *vcpu, u32 pvr)
>  /* Dummy value used in computing PCR value below */
>  #define PCR_ARCH_31    (PCR_ARCH_300 << 1)
>  
> +static inline unsigned long map_pcr_to_cap(unsigned long pcr)
> +{
> +	unsigned long cap = 0;
> +
> +	switch (pcr) {
> +	case PCR_ARCH_300:
> +		cap = H_GUEST_CAP_POWER9;
> +		break;
> +	case PCR_ARCH_31:
> +		cap = H_GUEST_CAP_POWER10;
> +	default:
> +		break;
> +	}
> +
> +	return cap;
> +}
> +
>  static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
>  {
>  	unsigned long host_pcr_bit = 0, guest_pcr_bit = 0, cap = 0;
> @@ -424,11 +441,9 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
>  			break;
>  		case PVR_ARCH_300:
>  			guest_pcr_bit = PCR_ARCH_300;
> -			cap = H_GUEST_CAP_POWER9;
>  			break;
>  		case PVR_ARCH_31:
>  			guest_pcr_bit = PCR_ARCH_31;
> -			cap = H_GUEST_CAP_POWER10;
>  			break;
>  		default:
>  			return -EINVAL;
> @@ -440,6 +455,12 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
>  		return -EINVAL;
>  
>  	if (kvmhv_on_pseries() && kvmhv_is_nestedv2()) {
> +		/*
> +		 * 'arch_compat == 0' would mean the guest should default to
> +		 * L1's compatibility. In this case, the guest would pick
> +		 * host's PCR and evaluate the corresponding capabilities.
> +		 */
> +		cap = map_pcr_to_cap(guest_pcr_bit);
>  		if (!(cap & nested_capabilities))
>  			return -EINVAL;
>  	}
> diff --git a/arch/powerpc/kvm/book3s_hv_nestedv2.c b/arch/powerpc/kvm/book3s_hv_nestedv2.c
> index 5378eb40b162..6042bdc70230 100644
> --- a/arch/powerpc/kvm/book3s_hv_nestedv2.c
> +++ b/arch/powerpc/kvm/book3s_hv_nestedv2.c
> @@ -138,6 +138,7 @@ static int gs_msg_ops_vcpu_fill_info(struct kvmppc_gs_buff *gsb,
>  	vector128 v;
>  	int rc, i;
>  	u16 iden;
> +	u32 arch_compat = 0;
>  
>  	vcpu = gsm->data;
>  
> @@ -347,8 +348,26 @@ static int gs_msg_ops_vcpu_fill_info(struct kvmppc_gs_buff *gsb,
>  			break;
>  		}
>  		case KVMPPC_GSID_LOGICAL_PVR:
> -			rc = kvmppc_gse_put_u32(gsb, iden,
> -						vcpu->arch.vcore->arch_compat);
> +			/*
> +			 * Though 'arch_compat == 0' would mean the default
> +			 * compatibility, arch_compat, being a Guest Wide
> +			 * Element, cannot be filled with a value of 0 in GSB
> +			 * as this would result into a kernel trap.
> +			 * Hence, when `arch_compat == 0`, arch_compat should
> +			 * default to L1's PVR.
> +			 *
> +			 * Rework this when PowerVM supports a value of 0
> +			 * for arch_compat for KVM API v2.
> +			 */
> +			if (!vcpu->arch.vcore->arch_compat) {
> +				if (cpu_has_feature(CPU_FTR_ARCH_31))
> +					arch_compat = PVR_ARCH_31;
> +				else if (cpu_has_feature(CPU_FTR_ARCH_300))
> +					arch_compat = PVR_ARCH_300;
> +			} else {
> +				arch_compat = vcpu->arch.vcore->arch_compat;
> +			}
> +			rc = kvmppc_gse_put_u32(gsb, iden, arch_compat);
>  			break;
>  		}
>  
>
> base-commit: 6764c317b6bb91bd806ef79adf6d9c0e428b191e
> -- 
> 2.43.0