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

[Greg Kurz <groug@kaod.org>]

On Fri, 11 Sep 2020 09:45:36 +0200
Greg Kurz <groug@kaod.org> wrote:

> On Fri, 11 Sep 2020 01:16:07 -0300
> Fabiano Rosas <farosas@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@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@linux.vnet.ibm.com>
> > Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
> > ---
> 
> LGTM
> 
> Reviewed-by: Greg Kurz <groug@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;

According to POSIX [1]:

[ENOTSUP]
Not supported. The implementation does not support the requested feature or value.

[EOPNOTSUPP]
Operation not supported on socket. The type of socket (address family or protocol) does not support the requested operation. A conforming implementation may assign the same values for [EOPNOTSUPP] and [ENOTSUP].

Even if these two happen to be equal in linux, it seems that ENOTSUP, which
doesn't refer to sockets, is more appropriate.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html

> > +			break;
> > +		}
> > +
> >  		r = -EFAULT;
> >  		if (get_user(htab_order, (u32 __user *)argp))
> >  			break;
>