[Virtio-fs] Failure as Operation not permitted on aarch64 machine

[Masayoshi Mizuma]

Hi Dave,

On 7/22/19 12:47 PM, Dr. David Alan Gilbert wrote:
> * Masayoshi Mizuma (msys.mizuma at gmail.com) wrote:
>> Hello,
> 
> Hi Masa,
> 
>> I would appreciate if you could help me to resolve following failure.
>>
>> I tried to use virtio-fs on aarch64 [1], however, qemu [2] failed to boot
>> as Operation not permitted.
>>
>>   ---
>>   # ./virtiofsd -o vhost_user_socket=/tmp/vhostqemu -o source=/tmp/share -o cache=none &
>>
>>   # qemu-system-aarch64 -machine virt,accel=kvm,gic-version=3 -cpu host -smp 8 ... [3]
>>
>>   UEFI firmware starting.
>>   kvm_set_phys_mem: error registering slot: Operation not permitted
>>   ---
>>
>> >From the ftrace log, the error happened because kvm_set_user_memory_region()
>> returned as -1 (EPERM).
>>
>>   ---
>>   qemu-system-aar-28381 [003] ....  6683.601097: tracing_mark_write: kvm_set_user_memory Slot#3 flags=0x0 gpa=0x8000000000 size=0x40000000 ua=0xfffe0ba00000 ret=-1
>>   ---
>>
>> kvm_set_user_memory_region() returned -1 because kvm_arch_prepare_memory_region()
>> in kernel returned as -EPERM.
> 
> Right, yes we know this is a problem on aarch64;  for aarch we disable
> the DAX mode;  if you check out our -dev branches and apply the
> following hack to the kernel:
> 
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 3f3c018571ee..60a9724f9c71 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -541,7 +541,7 @@ static int virtio_fs_setup_dax(struct virtio_device *vdev, struct virtio_fs *fs)
>                                            (u8)VIRTIO_FS_SHMCAP_ID_CACHE);
>         if (!have_cache) {
>                 dev_err(&vdev->dev, "%s: No cache capability\n", __func__);
> -               return -ENXIO;
> +               return 0;
>         } else {
>                 dev_notice(&vdev->dev, "Cache len: 0x%llx @ 0x%llx\n",
>                            cache_reg.len, cache_reg.addr);
> 
> and start the qemu with:
> -device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=myfs,cache-size=0
> 
> it should start up for you.

Thanks and the cache-size=0 option works for me!
qemu boots successfully and I can mount the myfs.

Thanks!
Masa

> 
>>    ---
>>    int kvm_arch_prepare_memory_region(struct kvm *kvm,
>>                                    struct kvm_memory_slot *memslot,
>>                                    const struct kvm_userspace_memory_region *mem,
>>                                    enum kvm_mr_change change)
>>    {
>>    ...
>>                    if (writable && !(vma->vm_flags & VM_WRITE)) {
>>                            ret = -EPERM;
>>                            break;
>>                    }
>>    ---
>>
>> Actually, the /proc/PID/maps showed ua=0xfffe0ba00000 didn't have PROT_WRITE.
>>
>>   ---
>>   ...
>>   014b0000-014f0000 rw-p 00000000 00:00 0 
>>   1c840000-1d470000 rw-p 00000000 00:00 0                                  [heap]
>>   fffe0ba00000-fffe4ba00000 ---p 00000000 00:00 0 <===
>>   fffe4ba00000-fffe4ba10000 ---p 00000000 00:00 0 
>>   fffe4bc00000-fffe4fc00000 rw-p 00000000 00:00 0 
>>   fffe4fc00000-fffe4fc10000 ---p 00000000 00:00 0 
>>   fffe4fe00000-fffe53e00000 rw-p 00000000 00:00 0 
>>   fffe53e00000-fffe53e10000 ---p 00000000 00:00 0 
>>   ...
>>   ---
>>
>> I'm not sure why ua=0xfffe0ba00000 didn't have PROT_WRITE because the memory
>> was allocated by qemu_anon_ram_alloc() and it should set PROT_READ | PROT_WRITE.
> 
> Because we explicitly mprotect it later (and in current versions we
> allocate with PROT_NONE) - on x86 this works fine, but aarch doesn't
> like it; we've not quite figured the rules why yet.
> 
> Dave
> 
>>
>>   ---
>>   qemu-system-aar-28372 [022] ....  6674.795027: tracing_mark_write: qemu_anon_ram_alloc size 1073741824 ptr 0xfffe0ba00000
>>   ---
>>
>> qemu boots successfully if I remove "-device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=myfs",
>> so I suppose the option is related to the failure...
>>
>>
>> [1]: host kernel is 5.2.0.
>>      guest kernel is https://github.com/rhvgoyal/linux/, branch: virtio-fs-dev-5.1.
>>
>> [2]: I got the qemu from:
>>      https://gitlab.com/virtio-fs/qemu.git
>>
>> [3]: Qemu option is:
>>
>> $QEMU -machine virt,accel=kvm,gic-version=3 \
>> 	-cpu host \
>> 	-smp 8 \
>>         -m 4G,maxmem=4G \
>> 	-object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on \
>> 	-numa node,memdev=mem \
>>         -drive file=/usr/share/AAVMF/AAVMF_CODE.fd,if=pflash,format=raw,unit=0,readonly=on \
>>         -drive file=$VARS,if=pflash,format=raw,unit=1 \
>> 	-chardev socket,id=char0,path=/tmp/vhostqemu \
>> 	-device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=myfs \
>> 	-nographic \
>> 	-serial mon:stdio \
>> 	--trace events=/tmp/qemu-trace-events \
>> 	-drive if=virtio,file=/var/lib/libvirt/images/guest.qcow2
>>
>> Thanks,
>> Masa
>>
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> --
> Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
>