Re: [PATCH 0/9] xen/x86: PVH Dom0 fixes and fallout adjustments

[Jan Beulich <jbeulich@suse.com>]

On 14.09.2021 13:15, Roger Pau Monné wrote:
> On Tue, Sep 14, 2021 at 11:03:23AM +0200, Jan Beulich wrote:
>> On 14.09.2021 10:32, Roger Pau Monné wrote:
>>> On Tue, Sep 07, 2021 at 12:04:34PM +0200, Jan Beulich wrote:
>>>> In order to try to debug hypervisor side breakage from XSA-378 I found
>>>> myself urged to finally give PVH Dom0 a try. Sadly things didn't work
>>>> quite as expected. In the course of investigating these issues I actually
>>>> spotted one piece of PV Dom0 breakage as well, a fix for which is also
>>>> included here.
>>>>
>>>> There are two immediate remaining issues (also mentioned in affected
>>>> patches):
>>>>
>>>> 1) It is not clear to me how PCI device reporting is to work. PV Dom0
>>>>    reports devices as they're discovered, including ones the hypervisor
>>>>    may not have been able to discover itself (ones on segments other
>>>>    than 0 or hotplugged ones). The respective hypercall, however, is
>>>>    inaccessible to PVH Dom0. Depending on the answer to this, either
>>>>    the hypervisor will need changing (to permit the call) or patch 2
>>>>    here will need further refinement.
>>>
>>> I would rather prefer if we could limit the hypercall usage to only
>>> report hotplugged segments to Xen. Then Xen would have to scan the
>>> segment when reported and add any devices found.
>>>
>>> Such hypercall must be used before dom0 tries to access any device, as
>>> otherwise the BARs won't be mapped in the second stage translation and
>>> the traps for the MCFG area won't be setup either.
>>
>> This might work if hotplugging would only ever be of segments, and not
>> of individual devices. Yet the latter is, I think, a common case (as
>> far as hotplugging itself is "common").
> 
> Right, I agree to use hypercalls to report either hotplugged segments
> or devices. However I would like to avoid mandating usage of the
> hypercall for non-hotplug stuff, as then OSes not having hotplug
> support don't really need to care about making use of those
> hypercalls.
> 
>> Also don't forget about SR-IOV VFs - they would typically not be there
>> when booting. They would materialize when the PF driver initializes
>> the device. This is, I think, something that can be dealt with by
>> intercepting writes to the SR-IOV capability.
> 
> My plan was to indeed trap SR-IOV capability accesses, see:
> 
> https://lore.kernel.org/xen-devel/20180717094830.54806-1-roger.pau@citrix.com/
> 
> I just don't have time ATM to continue this work.
> 
>> But I wonder whether
>> there might be other cases where devices become "visible" only while
>> the Dom0 kernel is already running.
> 
> I would consider those kind of hotplug devices, and hence would
> require the use of the hypercall in order to notify Xen about them.

So what does this mean for the one patch? Should drivers/xen/pci.c
then be built for PVH (and then have logic added to filter boot
time device discovery), or should I restrict this to be PV-only (and
PVH would get some completely different logic added later)?

>>>> 2) Dom0, unlike in the PV case, cannot access the screen (to use as a
>>>>    console) when in a non-default mode (i.e. not 80x25 text), as the
>>>>    necessary information (in particular about VESA-bases LFB modes) is
>>>>    not communicated. On the hypervisor side this looks like deliberate
>>>>    behavior, but it is unclear to me what the intentions were towards
>>>>    an alternative model. (X may be able to access the screen depending
>>>>    on whether it has a suitable driver besides the presently unusable
>>>>    /dev/fb<N> based one.)
>>>
>>> I had to admit most of my boxes are headless servers, albeit I have
>>> one NUC I can use to test gfx stuff, so I don't really use gfx output
>>> with Xen.
>>>
>>> As I understand such information is fetched from the BIOS and passed
>>> into Xen, which should then hand it over to the dom0 kernel?
>>
>> That's how PV Dom0 learns of the information, yes. See
>> fill_console_start_info(). (I'm in the process of eliminating the
>> need for some of the "fetch from BIOS" in Xen right now, but that's
>> not going to get us as far as being able to delete that code, no
>> matter how much in particular Andrew would like that to happen.)
>>
>>> I guess the only way for Linux dom0 kernel to fetch that information
>>> would be to emulate the BIOS or drop into realmode and issue the BIOS
>>> calls?
>>
>> Native Linux gets this information passed from the boot loader, I think
>> (except in the EFI case, as per below).
>>
>>> Is that an issue on UEFI also, or there dom0 can fetch the framebuffer
>>> info using the PV EFI interface?
>>
>> There it's EFI boot services functions which can be invoked before
>> leaving boot services (in the native case). Aiui the PVH entry point
>> lives logically past any EFI boot services interaction, and hence
>> using them is not an option (if there was EFI firmware present in Dom0
>> in the first place, which I consider difficult all by itself - this
>> can't be the physical system's firmware, but I also don't see where
>> virtual firmware would be taken from).
>>
>> There is no PV EFI interface to obtain video information. With the
>> needed information getting passed via start_info, PV has no need for
>> such, and I would be hesitant to add a fundamentally redundant
>> interface for PVH. The more that the information needed isn't EFI-
>> specific at all.
> 
> I think our only option is to expand the HVM start info information to
> convey that data from Xen into dom0.

PHV doesn't use the ordinary start_info, does it?

Jan