Re: [PATCH v6 10/13] vpci/header: reset the command register when adding devices

From: Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "julien@xen.org" <julien@xen.org>,
	"sstabellini@kernel.org" <sstabellini@kernel.org>,
	Oleksandr Tyshchenko <Oleksandr_Tyshchenko@epam.com>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	Artem Mygaiev <Artem_Mygaiev@epam.com>,
	"roger.pau@citrix.com" <roger.pau@citrix.com>,
	"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
	"george.dunlap@citrix.com" <george.dunlap@citrix.com>,
	"paul@xen.org" <paul@xen.org>,
	Bertrand Marquis <bertrand.marquis@arm.com>,
	Rahul Singh <rahul.singh@arm.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>
Subject: Re: [PATCH v6 10/13] vpci/header: reset the command register when adding devices
Date: Mon, 7 Feb 2022 12:51:35 +0000	[thread overview]
Message-ID: <fbd410d1-84ad-2309-ca05-b9ac7f95f1f9@epam.com> (raw)
In-Reply-To: <49380584-0577-96f2-ef1c-a96eabf0d227@suse.com>


On 07.02.22 14:38, Jan Beulich wrote:
> On 07.02.2022 12:27, Oleksandr Andrushchenko wrote:
>>
>> On 07.02.22 09:29, Jan Beulich wrote:
>>> On 04.02.2022 15:37, Oleksandr Andrushchenko wrote:
>>>> On 04.02.22 16:30, Jan Beulich wrote:
>>>>> On 04.02.2022 07:34, Oleksandr Andrushchenko wrote:
>>>>>> Reset the command register when assigning a PCI device to a guest:
>>>>>> according to the PCI spec the PCI_COMMAND register is typically all 0's
>>>>>> after reset.
>>>>> It's not entirely clear to me whether setting the hardware register to
>>>>> zero is okay. What wants to be zero is the value the guest observes
>>>>> initially.
>>>> "the PCI spec says the PCI_COMMAND register is typically all 0's after reset."
>>>> Why wouldn't it be ok? What is the exact concern here?
>>> The concern is - as voiced is similar ways before, perhaps in other
>>> contexts - that you need to consider bit-by-bit whether overwriting
>>> with 0 what is currently there is okay. Xen and/or Dom0 may have put
>>> values there which they expect to remain unaltered. I guess
>>> PCI_COMMAND_SERR is a good example: While the guest's view of this
>>> will want to be zero initially, the host having set it to 1 may not
>>> easily be overwritten with 0, or else you'd effectively imply giving
>>> the guest control of the bit.
>> We have already discussed in great detail PCI_COMMAND emulation [1].
>> At the end you wrote [1]:
>> "Well, in order for the whole thing to be security supported it needs to
>> be explained for every bit why it is safe to allow the guest to drive it.
>> Until you mean vPCI to reach that state, leaving TODO notes in the code
>> for anything not investigated may indeed be good enough.
>>
>> Jan"
>>
>> So, this is why I left a TODO in the PCI_COMMAND emulation for now and only
>> care about INTx which is honored with the code in this patch.
> Right. The issue I see is that the description does not have any
> mention of this, but instead talks about simply writing zero.
How do you want that mentioned? Extended commit message or
just a link to the thread [1]?
With the above done, do you think that writing 0's is an acceptable
approach as of now?
> Jan
>
Thank you,
Oleksandr