Re: [PATCH 0/7] xen/arm: Emulate ID registers

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/11/2020 10:20, Bertrand Marquis wrote:
> Hi Andrew,
>
>> On 27 Nov 2020, at 20:07, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>
>> On 26/11/2020 15:51, Bertrand Marquis wrote:
>>> The goal of this serie is to emulate coprocessor ID registers so that
>>> Xen only publish to guest features that are supported by Xen and can
>>> actually be used by guests.
>>> One practical example where this is required are SVE support which is
>>> forbidden by Xen as it is not supported, but if Linux is compiled with
>>> it, it will crash on boot. An other one is AMU which is also forbidden
>>> by Xen but one Linux compiled with it would crash if the platform
>>> supports it.
>>>
>>> To be able to emulate the coprocessor registers defining what features
>>> are supported by the hardware, the TID3 bit of HCR must be disabled and
>>> Xen must emulated the values of those registers when an exception is
>>> catched when a guest is accessing it.
>>>
>>> This serie is first creating a guest cpuinfo structure which will
>>> contain the values that we want to publish to the guests and then
>>> provides the proper emulationg for those registers when Xen is getting
>>> an exception due to an access to any of those registers.
>>>
>>> This is a first simple implementation to solve the problem and the way
>>> to define the values that we provide to guests and which features are
>>> disabled will be in a future patchset enhance so that we could decide
>>> per guest what can be used or not and depending on this deduce the bits
>>> to activate in HCR and the values that we must publish on ID registers.
>>>
>>> Bertrand Marquis (7):
>>>  xen/arm: Add ID registers and complete cpufinfo
>>>  xen/arm: Add arm64 ID registers definitions
>>>  xen/arm: create a cpuinfo structure for guest
>>>  xen/arm: Add handler for ID registers on arm64
>>>  xen/arm: Add handler for cp15 ID registers
>>>  xen/arm: Add CP10 exception support to handle VMFR
>>>  xen/arm: Activate TID3 in HCR_EL2
>> CI found an ARM randconfig failure against this series.
>>
>> https://gitlab.com/xen-project/patchew/xen/-/pipelines/221798884
>>
>> I have admit that I can't spot an obvious connection so it might be
>> collateral damage from elsewhere, but does need looking at irrespective.
> This absolutely right, there is a bug in my code and i will send a V2 to fix 
> it.
>
> Very nice finding, i am wondering why my tests did not point this out.

Its randconfig, so every time the test runs, it picks a new random
Kconfig configuration.

Sadly, it is non-deterministic, and not necessarily the fault of change
the test ran against.  We're probably going to have to tweak how we run
these tests before the CI goes too much further.

~Andrew