Re: [Xen-devel] [PATCH v3 07/15] libxl: disallow attaching the same device more than once

["Li, Liang Z" <liang.z.li@xxxxxxxxx>]

> Originally the code allowed users to attach the same device more than
> once. It just stupidly overwrites xenstore entries. This is bogus as
> frontend will be very confused.
>
> Introduce a helper function to check if the device to be written to
> xenstore already exists. A new error code is also introduced.
>
> The check and add are within one xs transaction.
>
> Signed-off-by: Wei Liu <wei....@xxxxxxxxxx>
> ---

I find this patch will cause the pci-attach failed if more than one virtual 
function devices are attached to the guest.


>  @@ -148,15 +150,32 @@ static int libxl__device_pci_add_xenstore(libxl__gc 
> *gc,
>  uint32_t domid, libxl_d
>       if (!starting)
>           flexarray_append_pair(back, "state", libxl__sprintf(gc, "%d", 7));
>  
>  -retry_transaction:
>  -    t = xs_transaction_start(ctx->xsh);
>  -    libxl__xs_writev(gc, t, be_path,
>  -                    libxl__xs_kvs_of_flexarray(gc, back, back->count));
>  -    if (!xs_transaction_end(ctx->xsh, t, 0))
>  -        if (errno == EAGAIN)
>  -            goto retry_transaction;
>  +    GCNEW(device);
>  +    libxl__device_from_pcidev(gc, domid, pcidev, device);

>  -    return 0;
>  +    for (;;) {
>  +        rc = libxl__xs_transaction_start(gc, &t);
>  +        if (rc) goto out;
>  +
>  +        rc = libxl__device_exists(gc, t, device);
>  +        if (rc < 0) goto out;
>  +        if (rc == 1) {
>  +            LOG(ERROR, "device already exists in xenstore");
>  +            rc = ERROR_DEVICE_EXISTS;
>  +            goto out;
>  +        } 

The libxl__device_exists will return 1 if more than one PCI devices are 
attached to the guest, no matter the BDFs are identical or not.
I don't understand why to check this condition here, if the same device was 
attached more than once the xc_test_assign_device() will return error,
and the libxl__device_pci_add_xenstore() will not be called. It seems 
unnecessary.



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