Re: [PATCH v3 2/4] KVM: X86: Fix loss of exception which has not yet injected

[Ross Zwisler]

On Wed, Aug 23, 2017 at 10:21 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> vmx_complete_interrupts() assumes that the exception is always injected,
> so it would be dropped by kvm_clear_exception_queue(). This patch separates
> exception.pending from exception.injected, exception.inject represents the
> exception is injected or the exception should be reinjected due to vmexit
> occurs during event delivery in VMX non-root operation. exception.pending
> represents the exception is queued and will be cleared when injecting the
> exception to the guest. So exception.pending and exception.injected can
> cooperate to guarantee exception will not be lost.
>
> Reported-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---

I'm seeing a regression in my QEMU based NVDIMM testing system, and I
bisected it to this commit.

The behavior I'm seeing is that heavy I/O to simulated NVDIMMs in
multiple virtual machines causes the QEMU guests to receive double
faults, crashing them.  Here's an example backtrace:

[ 1042.653816] PANIC: double fault, error_code: 0x0
[ 1042.654398] CPU: 2 PID: 30257 Comm: fsstress Not tainted 4.15.0-rc5 #1
[ 1042.655169] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.10.2-2.fc27 04/01/2014
[ 1042.656121] RIP: 0010:memcpy_flushcache+0x4d/0x180
[ 1042.656631] RSP: 0018:ffffac098c7d3808 EFLAGS: 00010286
[ 1042.657245] RAX: ffffac0d18ca8000 RBX: 0000000000000fe0 RCX: ffffac0d18ca8000
[ 1042.658085] RDX: ffff921aaa5df000 RSI: ffff921aaa5e0000 RDI: 000019f26e6c9000
[ 1042.658802] RBP: 0000000000001000 R08: 0000000000000000 R09: 0000000000000000
[ 1042.659503] R10: 0000000000000000 R11: 0000000000000000 R12: ffff921aaa5df020
[ 1042.660306] R13: ffffac0d18ca8000 R14: fffff4c102a977c0 R15: 0000000000001000
[ 1042.661132] FS:  00007f71530b90c0(0000) GS:ffff921b3b280000(0000)
knlGS:0000000000000000
[ 1042.662051] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1042.662528] CR2: 0000000001156002 CR3: 000000012a936000 CR4: 00000000000006e0
[ 1042.663093] Call Trace:
[ 1042.663329]  write_pmem+0x6c/0xa0 [nd_pmem]
[ 1042.663668]  pmem_do_bvec+0x15f/0x330 [nd_pmem]
[ 1042.664056]  ? kmem_alloc+0x61/0xe0 [xfs]
[ 1042.664393]  pmem_make_request+0xdd/0x220 [nd_pmem]
[ 1042.664781]  generic_make_request+0x11f/0x300
[ 1042.665135]  ? submit_bio+0x6c/0x140
[ 1042.665436]  submit_bio+0x6c/0x140
[ 1042.665754]  ? next_bio+0x18/0x40
[ 1042.666025]  ? _cond_resched+0x15/0x40
[ 1042.666341]  submit_bio_wait+0x53/0x80
[ 1042.666804]  blkdev_issue_zeroout+0xdc/0x210
[ 1042.667336]  ? __dax_zero_page_range+0xb5/0x140
[ 1042.667810]  __dax_zero_page_range+0xb5/0x140
[ 1042.668197]  ? xfs_file_iomap_begin+0x2bd/0x8e0 [xfs]
[ 1042.668611]  iomap_zero_range_actor+0x7c/0x1b0
[ 1042.668974]  ? iomap_write_actor+0x170/0x170
[ 1042.669318]  iomap_apply+0xa4/0x110
[ 1042.669616]  ? iomap_write_actor+0x170/0x170
[ 1042.669958]  iomap_zero_range+0x52/0x80
[ 1042.670255]  ? iomap_write_actor+0x170/0x170
[ 1042.670616]  xfs_setattr_size+0xd4/0x330 [xfs]
[ 1042.670995]  xfs_ioc_space+0x27e/0x2f0 [xfs]
[ 1042.671332]  ? terminate_walk+0x87/0xf0
[ 1042.671662]  xfs_file_ioctl+0x862/0xa40 [xfs]
[ 1042.672035]  ? _copy_to_user+0x22/0x30
[ 1042.672346]  ? cp_new_stat+0x150/0x180
[ 1042.672663]  do_vfs_ioctl+0xa1/0x610
[ 1042.672960]  ? SYSC_newfstat+0x3c/0x60
[ 1042.673264]  SyS_ioctl+0x74/0x80
[ 1042.673661]  entry_SYSCALL_64_fastpath+0x1a/0x7d
[ 1042.674239] RIP: 0033:0x7f71525a2dc7
[ 1042.674681] RSP: 002b:00007ffef97aa778 EFLAGS: 00000246 ORIG_RAX:
0000000000000010
[ 1042.675664] RAX: ffffffffffffffda RBX: 00000000000112bc RCX: 00007f71525a2dc7
[ 1042.676592] RDX: 00007ffef97aa7a0 RSI: 0000000040305825 RDI: 0000000000000003
[ 1042.677520] RBP: 0000000000000009 R08: 0000000000000045 R09: 00007ffef97aa78c
[ 1042.678442] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
[ 1042.679330] R13: 0000000000019e38 R14: 00000000000fcca7 R15: 0000000000000016
[ 1042.680216] Code: 48 8d 5d e0 4c 8d 62 20 48 89 cf 48 29 d7 48 89
de 48 83 e6 e0 4c 01 e6 48 8d 04 17 4c 8b 02 4c 8b 4a 08 4c 8b 52 10
4c 8b 5a 18 <4c> 0f c3 00 4c 0f c3 48 08 4c 0f c3 50 10 4c 0f c3 58 18
48 83

This appears to be independent of both the guest kernel version (this
backtrace has v4.15.0-rc5, but I've seen it with other kernels) as
well as independent of the host QMEU version (mine happens to be
qemu-2.10.1-2.fc27 in Fedora 27).

The new behavior is due to this commit being present in the host OS
kernel.  Prior to this commit I could fire up 4 VMs and run xfstests
on my simulated NVDIMMs, but after this commit such testing results in
multiple of my VMs crashing almost immediately.

Reproduction is very simple, at least on my development box.  All you
need are a pair of VMs (I just did it with clean installs of Fedora
27) with NVDIMMs.  Here's a sample QEMU command to get one of these:

# qemu-system-x86_64 /home/rzwisler/vms/Fedora27.qcow2 -m
4G,slots=3,maxmem=512G -smp 12 -machine pc,accel=kvm,nvdimm
-enable-kvm -object
memory-backend-file,id=mem1,share,mem-path=/home/rzwisler/nvdimms/nvdimm-1,size=17G
-device nvdimm,memdev=mem1,id=nv1

In my setup my NVDIMMs backing files (/home/rzwisler/nvdimms/nvdimm-1)
are being created on a filesystem on an SSD.

After these two qemu guests are up, run write I/Os to the resulting
/dev/pmem0 devices.   I've done this with xfstests and fio to get the
error, but the simplest way is just:

# dd if=/dev/zero of=/dev/pmem0

The double fault should happen in under a minute, definitely before
the DDs run out of space on their /dev/pmem0 devices.

I've reproduced this on multiple development boxes, so I'm pretty sure
it's not related to a flakey hardware setup.

Thanks,
- Ross