Re: [PATCH v5 3/3] ivshmem: add check on protocol version in QEMU

[David Marchand <david.marchand@6wind.com>]

Hello Stefan,

On 09/05/2014 12:29 PM, Stefan Hajnoczi wrote:
> On Thu, Sep 04, 2014 at 02:51:01PM +0200, David Marchand wrote:
>> diff --git a/contrib/ivshmem-client/ivshmem-client.c b/contrib/ivshmem-client/ivshmem-client.c
>> index ad210c8..0c4e016 100644
>> --- a/contrib/ivshmem-client/ivshmem-client.c
>> +++ b/contrib/ivshmem-client/ivshmem-client.c
>> @@ -184,10 +184,18 @@ ivshmem_client_connect(IvshmemClient *client)
>>           goto err_close;
>>       }
>>
>> -    /* first, we expect our index + a fd == -1 */
>> +    /* first, we expect a protocol version */
>> +    if (read_one_msg(client, &tmp, &fd) < 0 ||
>> +        (tmp != IVSHMEM_PROTOCOL_VERSION) || fd != -1) {
>> +        debug_log(client, "cannot read from server\n");
>> +        goto err_close;
>> +    }
>> +    debug_log(client, "our_id=%ld\n", client->local.id);
>
> This debug_log() is probably not intentional.  local.id will always be
> -1 here so the output is not useful.

Yes, this is most likely a merge/rebase issue.
Will remove this.

>
>> +static void ivshmem_check_version(void *opaque, const uint8_t * buf, int flags)
>> +{
>> +    IVShmemState *s = opaque;
>> +    PCIDevice *dev = PCI_DEVICE(s);
>> +    int tmp;
>> +    long version;
>> +
>> +    memcpy(&version, buf, sizeof(long));
>> +    tmp = qemu_chr_fe_get_msgfd(s->server_chr);
>> +    if (tmp != -1 || version != IVSHMEM_PROTOCOL_VERSION) {
>> +        fprintf(stderr, "incompatible version, you are connecting to a ivhsmem-"

Hum, typo: ivhs -> ivsh.

>> +                "server using a different protocol please check your setup\n");
>> +        qemu_chr_delete(s->server_chr);
>> +        s->server_chr = NULL;
>> +        return;
>> +    }
>> +
>> +    IVSHMEM_DPRINTF("version check ok, finish init and switch to real chardev "
>> +                    "handler\n");
>> +
>> +    pci_register_bar(dev, 2, s->ivshmem_attr, &s->bar);
>
> Not sure if it is okay to delay PCI initialization to a fd hander
> callback.
>
> If the version message is too slow the guest could see the PCI adapter
> without the BAR!
>
> Did you move this code in order to prevent the guest from accessing the
> device before it has connected to the server?  Perhaps the device needs
> a state field that tracks whether or not it is ready for operation.  Any
> access before RUNNING state is reached will be ignored (?).

Yes, exactly.

There already is a synchronisation mechanism described in the documentation:
"When using the server, since the server is a separate process, the VM 
ID will only be set when the device is ready (shared memory is received 
from the server and accessible via the device).  If the device is not 
ready, the IVPosition will return -1.
Applications should ensure that they have a valid VM ID before accessing 
the shared memory."

So actually, this move is unneeded if ivshmem users comply to this.

I will let the init stuff (pci_register_bar + gmalloc) where it was 
before, ivshmem_check_version will only switch the chardev handler.

What do you think about this ?


Thanks.

-- 
David Marchand