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From: Laszlo Ersek <lersek@redhat.com>
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Date: Thu, 27 Jul 2017 01:11:22 +0200
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Subject: Re: [Qemu-devel] [SeaBIOS] [RFC PATCH v2 4/6] hw/pci: introduce
 bridge-only vendor-specific capability to provide some hints to firmware
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To: Alexander Bezzubikov <zuban32s@gmail.com>, "Michael S. Tsirkin" <mst@redhat.com>
Cc: seabios@seabios.org, qemu-devel@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>, pbonzini@redhat.com, Marcel Apfelbaum <marcel@redhat.com>, rth@twiddle.net

On 07/26/17 23:54, Alexander Bezzubikov wrote:
> 2017-07-26 22:43 GMT+03:00 Michael S. Tsirkin <mst@redhat.com>:
>> On Sun, Jul 23, 2017 at 01:15:41AM +0300, Aleksandr Bezzubikov wrote:

>>> +    PCIBridgeQemuCap cap;
>>
>> This leaks info to guest. You want to init all fields here:
>>
>> cap = {
>>  .len = ....
>> };
> 
> I surely can do this for len field, but as Laszlo proposed
> we can use mutually exclusive fields,
> e.g. pref_32 and pref_64, the only way I have left
> is to use ternary operator (if we surely need this
> big initializer). Keeping some if's would look better,
> I think.

I think it's fine to use "if"s in order to set up the structure
partially / gradually, but then please clear the structure up-front:


  PCIBridgeQemuCap cap = { 0 };

(In general "{ 0 }" is the best initializer ever, because it can
zero-init a variable of *any* type at all. Gcc might complain about the
inexact depth of {} nesting of course, but it's nonetheless valid C.)

Or else add a memset-to-zero.

Or else, do just

  PCIBridgeQemuCap cap = { .len = ... };

which will zero-fill every other field. ("[...] all subobjects that are
not initialized explicitly shall be initialized implicitly the same as
objects that have static storage duration").

Thanks
Laszlo