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Subject: Re: [PATCH v2 1/4] usb: early: Avoid using DbC if already enabled
To:     Connor Davis <connojdavis@gmail.com>,
        Jan Beulich <jbeulich@suse.com>
Cc:     Jann Horn <jannh@google.com>, Lee Jones <lee.jones@linaro.org>,
        Chunfeng Yun <chunfeng.yun@mediatek.com>,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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From:   Mathias Nyman <mathias.nyman@linux.intel.com>
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Message-ID: <0dac96cc-dce1-e908-400e-9c8e68c41a54@linux.intel.com>
Date:   Wed, 19 May 2021 00:50:46 +0300
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Precedence: bulk

On 17.5.2021 17.24, Connor Davis wrote:
> 
> On 5/17/21 8:13 AM, Jan Beulich wrote:
>> On 17.05.2021 15:48, Connor Davis wrote:
>>> On 5/17/21 3:32 AM, Jan Beulich wrote:
>>>> On 14.05.2021 02:56, Connor Davis wrote:
>>>>> Check if the debug capability is enabled in early_xdbc_parse_parameter,
>>>>> and if it is, return with an error. This avoids collisions with whatever
>>>>> enabled the DbC prior to linux starting.
>>>> Doesn't this go too far and prevent use even if firmware (perhaps
>>>> mistakenly) left it enabled?
>>> Yes, but how is one supposed to distinguish the broken firmware and
>>> non-broken
>>>
>>> firmware cases?
>> Well, a first step might be to only check if running virtualized.
>> And then if your running virtualized, there might be a way to
>> inquire the hypervisor?
> 
> Right, but if it was enabled by something other than a hypervisor,
> 
> or you're not running virtualized, how do you distinguish then? IMO
> 
> the proper thing to do in any case is to simply not use the DbC in linux.
> 

Sounds reasonable.

We can address "broken firmware" during the xHC handoff from BIOS to OS. 
Only first OS that loads after BIOS should see the 
"HC BIOS owned semaphore" bit set in xHCI MMIO.

If it's set then OS requests ownership, which clears BIOS owned bit.
If DbC is running here we can stop it.

-Mathias