On Fri, 11 Sep 2020 10:08:10 +0200
Michal Suchánek <msuchanek@xxxxxxx> wrote:
> On Fri, Sep 11, 2020 at 10:01:33AM +0200, Greg Kurz wrote:
> > On Fri, 11 Sep 2020 09:45:36 +0200
> > Greg Kurz <groug@xxxxxxxx> wrote:
> >
> > > On Fri, 11 Sep 2020 01:16:07 -0300
> > > Fabiano Rosas <farosas@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Fabiano Rosas <farosas@xxxxxxxxxxxxx>
> > > > ---
> > >
> > > LGTM
> > >
> > > Reviewed-by: Greg Kurz <groug@xxxxxxxx>
> > >
> > > > 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
>
> Which in Linux kernel maps to a completely bogus value not recognized by
> userspace.
>
There's no such thing as ENOTSUP in Linux actually :)
Linux has ENOTSUPP which indeed doesn't map to anything usable by
userspace... very unfortunate naming :-(
> So when you mean ENOTSUP use EOPNOTSUPP in Linux.
>
Right. So this patch is good as it is.
Cheers,
--
Greg
> Thanks
>
> Michal
