[PATCH] KVM: PPC: Book3S HV: Do not allocate HPT for a nested guest

[Greg Kurz]

On Fri, 11 Sep 2020 01:16:07 -0300
Fabiano Rosas <farosas at linux.ibm.com> wrote:

> The current nested KVM code does not support HPT guests. This is
> informed/enforced in some ways:
> 
> - Hosts < P9 will not be able to enable the nested HV feature;
> 
> - The nested hypervisor MMU capabilities will not contain
>   KVM_CAP_PPC_MMU_HASH_V3;
> 
> - QEMU reflects the MMU capabilities in the
>   'ibm,arch-vec-5-platform-support' device-tree property;
> 
> - The nested guest, at 'prom_parse_mmu_model' ignores the
>   'disable_radix' kernel command line option if HPT is not supported;
> 
> - The KVM_PPC_CONFIGURE_V3_MMU ioctl will fail if trying to use HPT.
> 
> There is, however, still a way to start a HPT guest by using
> max-compat-cpu=power8 at the QEMU machine options. This leads to the
> guest being set to use hash after QEMU calls the KVM_PPC_ALLOCATE_HTAB
> ioctl.
> 
> With the guest set to hash, the nested hypervisor goes through the
> entry path that has no knowledge of nesting (kvmppc_run_vcpu) and
> crashes when it tries to execute an hypervisor-privileged (mtspr
> HDEC) instruction at __kvmppc_vcore_entry:
> 
> root at L1:~ $ qemu-system-ppc64 -machine pseries,max-cpu-compat=power8 ...
> 
> <snip>
> [  538.543303] CPU: 83 PID: 25185 Comm: CPU 0/KVM Not tainted 5.9.0-rc4 #1
> [  538.543355] NIP:  c00800000753f388 LR: c00800000753f368 CTR: c0000000001e5ec0
> [  538.543417] REGS: c0000013e91e33b0 TRAP: 0700   Not tainted  (5.9.0-rc4)
> [  538.543470] MSR:  8000000002843033 <SF,VEC,VSX,FP,ME,IR,DR,RI,LE>  CR: 22422882  XER: 20040000
> [  538.543546] CFAR: c00800000753f4b0 IRQMASK: 3
>                GPR00: c0080000075397a0 c0000013e91e3640 c00800000755e600 0000000080000000
>                GPR04: 0000000000000000 c0000013eab19800 c000001394de0000 00000043a054db72
>                GPR08: 00000000003b1652 0000000000000000 0000000000000000 c0080000075502e0
>                GPR12: c0000000001e5ec0 c0000007ffa74200 c0000013eab19800 0000000000000008
>                GPR16: 0000000000000000 c00000139676c6c0 c000000001d23948 c0000013e91e38b8
>                GPR20: 0000000000000053 0000000000000000 0000000000000001 0000000000000000
>                GPR24: 0000000000000001 0000000000000001 0000000000000000 0000000000000001
>                GPR28: 0000000000000001 0000000000000053 c0000013eab19800 0000000000000001
> [  538.544067] NIP [c00800000753f388] __kvmppc_vcore_entry+0x90/0x104 [kvm_hv]
> [  538.544121] LR [c00800000753f368] __kvmppc_vcore_entry+0x70/0x104 [kvm_hv]
> [  538.544173] Call Trace:
> [  538.544196] [c0000013e91e3640] [c0000013e91e3680] 0xc0000013e91e3680 (unreliable)
> [  538.544260] [c0000013e91e3820] [c0080000075397a0] kvmppc_run_core+0xbc8/0x19d0 [kvm_hv]
> [  538.544325] [c0000013e91e39e0] [c00800000753d99c] kvmppc_vcpu_run_hv+0x404/0xc00 [kvm_hv]
> [  538.544394] [c0000013e91e3ad0] [c0080000072da4fc] kvmppc_vcpu_run+0x34/0x48 [kvm]
> [  538.544472] [c0000013e91e3af0] [c0080000072d61b8] kvm_arch_vcpu_ioctl_run+0x310/0x420 [kvm]
> [  538.544539] [c0000013e91e3b80] [c0080000072c7450] kvm_vcpu_ioctl+0x298/0x778 [kvm]
> [  538.544605] [c0000013e91e3ce0] [c0000000004b8c2c] sys_ioctl+0x1dc/0xc90
> [  538.544662] [c0000013e91e3dc0] [c00000000002f9a4] system_call_exception+0xe4/0x1c0
> [  538.544726] [c0000013e91e3e20] [c00000000000d140] system_call_common+0xf0/0x27c
> [  538.544787] Instruction dump:
> [  538.544821] f86d1098 60000000 60000000 48000099 e8ad0fe8 e8c500a0 e9264140 75290002
> [  538.544886] 7d1602a6 7cec42a6 40820008 7d0807b4 <7d164ba6> 7d083a14 f90d10a0 480104fd
> [  538.544953] ---[ end trace 74423e2b948c2e0c ]---
> 
> This patch makes the KVM_PPC_ALLOCATE_HTAB ioctl fail when running in
> the nested hypervisor, causing QEMU to abort.
> 
> Reported-by: Satheesh Rajendran <sathnaga at linux.vnet.ibm.com>
> Signed-off-by: Fabiano Rosas <farosas at linux.ibm.com>
> ---

LGTM

Reviewed-by: Greg Kurz <groug at kaod.org>

>  arch/powerpc/kvm/book3s_hv.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 4ba06a2a306c..764b6239ef72 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -5250,6 +5250,12 @@ static long kvm_arch_vm_ioctl_hv(struct file *filp,
>  	case KVM_PPC_ALLOCATE_HTAB: {
>  		u32 htab_order;
>  
> +		/* If we're a nested hypervisor, we currently only support radix */
> +		if (kvmhv_on_pseries()) {
> +			r = -EOPNOTSUPP;
> +			break;
> +		}
> +
>  		r = -EFAULT;
>  		if (get_user(htab_order, (u32 __user *)argp))
>  			break;