Re: qemu-system-aarch64: Failed to retrieve host CPU features

[Marc Zyngier <maz@kernel.org>]

Hi Peter,

On Fri, 12 Aug 2022 10:25:55 +0100,
Peter Maydell <peter.maydell@linaro.org> wrote:
> 
> I've added some more relevant mailing lists to the cc.
> 
> On Fri, 12 Aug 2022 at 09:45, Vitaly Chikunov <vt@altlinux.org> wrote:
> > On Fri, Aug 12, 2022 at 05:14:27AM +0300, Vitaly Chikunov wrote:
> > > I noticed that we starting to get many errors like this:
> > >
> > >   qemu-system-aarch64: Failed to retrieve host CPU features
> > >
> > > Where many is 1-2% per run, depends on host, host is Kunpeng-920, and
> > > Linux kernel is v5.15.59, but it started to appear months before that.
> > >
> > > strace shows in erroneous case:
> > >
> > >   1152244 ioctl(9, KVM_CREATE_VM, 0x30)   = -1 EINTR (Interrupted system call)
> > >
> > > And I see in target/arm/kvm.c:kvm_arm_create_scratch_host_vcpu:
> > >
> > >     vmfd = ioctl(kvmfd, KVM_CREATE_VM, max_vm_pa_size);
> > >     if (vmfd < 0) {
> > >         goto err;
> > >     }
> > >
> > > Maybe it should restart ioctl on EINTR?
> > >
> > > I don't see EINTR documented in ioctl(2) nor in Linux'
> > > Documentation/virt/kvm/api.rst for KVM_CREATE_VM, but for KVM_RUN it
> > > says "an unmasked signal is pending".
> >
> > I am suggested that almost any blocking syscall could return EINTR, so I
> > checked the strace log and it does not show evidence of arriving a signal,
> > the log ends like this:
> >
> >   1152244 openat(AT_FDCWD, "/dev/kvm", O_RDWR|O_CLOEXEC) = 9
> >   1152244 ioctl(9, KVM_CHECK_EXTENSION, KVM_CAP_ARM_VM_IPA_SIZE) = 48
> >   1152244 ioctl(9, KVM_CREATE_VM, 0x30)   = -1 EINTR (Interrupted system call)
> >   1152244 close(9)                        = 0
> >   1152244 newfstatat(2, "", {st_dev=makedev(0, 0xd), st_ino=57869925, st_mode=S_IFIFO|0600, st_nlink=1, st_uid=517, st_gid=517, st_blksize=4096, st_blocks=0, st_size=0, st_atime=1660268019 /* 2022-08-12T01:33:39.850436293+0000 */, st_atime_nsec=850436293, st_mtime=1660268019 /* 2022-08-12T01:33:39.850436293+0000 */, st_mtime_nsec=850436293, st_ctime=1660268019 /* 2022-08-12T01:33:39.850436293+0000 */, st_ctime_nsec=850436293}, AT_EMPTY_PATH) = 0
> >   1152244 write(2, "qemu-system-aarch64: Failed to r"..., 58) = 58
> >   1152244 exit_group(1)                   = ?
> >   1152245 <... clock_nanosleep resumed> <unfinished ...>) = ?
> >   1152245 +++ exited with 1 +++
> >   1152244 +++ exited with 1 +++
> 
> KVM folks: should we expect that KVM_CREATE_VM might fail EINTR
> and need retrying?

In general, yes. But for this particular one, this is pretty odd.

The only path I can so far see that would match this behaviour is if
mm_take_all_locks() (called from __mmu_notifier_register()) was
getting interrupted by a signal (I'm looking at a 5.19-ish kernel,
which may slightly differ from the 5.15 mentioned above).

But as Vitaly points out, it doesn't seem to be a signal delivered
here.

Vitaly: could you please share your exact test case (full qemu command
line), and instrument your kernel to see if mm_take_all_locks() is the
one failing?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.
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