linux-kernel - Re: [PATCH 0/6] Introduce ID_PFR2 and other CPU feature changes

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Date:   Thu, 2 Apr 2020 08:03:09 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Peter Maydell <peter.maydell@...aro.org>
Cc:     arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH 0/6] Introduce ID_PFR2 and other CPU feature changes

On 02/14/2020 09:28 PM, Peter Maydell wrote:
> On Fri, 14 Feb 2020 at 04:23, Anshuman Khandual
> <anshuman.khandual@....com> wrote:
>>
>>
>>
>> On 01/28/2020 06:09 PM, Anshuman Khandual wrote:
>>> This series is primarily motivated from an adhoc list from Mark Rutland
>>> during our ID_ISAR6 discussion [1]. Besides, it also includes a patch
>>> which does macro replacement for various open bits shift encodings in
>>> various CPU ID registers. This series is based on linux-next 20200124.
>>>
>>> [1] https://patchwork.kernel.org/patch/11287805/
>>>
>>> Is there anything else apart from these changes which can be accommodated
>>> in this series, please do let me know. Thank you.
>>
>> Just a gentle ping. Any updates, does this series looks okay ? Is there
>> anything else related to CPU ID register feature bits, which can be added
>> up here. FWIW, the series still applies on v5.6-rc1.

Sorry for the delay in response, was distracted on some other patches.

> 
> I just ran into some "32-bit KVM doesn't expose all the ID
> registers to userspace via the ONE_REG API" issues today.
> I don't know if they'd be reasonable as something to include
> in this patchset or if they're unrelated.

IMHO, they are bit unrelated.

> 
> Anyway, missing stuff I have noticed specifically:
>  * MVFR2
>  * ID_MMFR4
>  * ID_ISAR6
> 
> More generally I would have expected all these 32-bit registers
> to exist and read-as-zero for the purpose of the ONE_REG APIs,
> because that's what the architecture says is supposed to happen
> and it means we have compatibility and QEMU doesn't gradually
> build up lots of "kernel doesn't support this yet" conditionals...
> I think we get this right for 64-bit KVM, but can we do it for
> 32-bit as well?

I am not very familiar with 32-bit KVM but will definitely keep these
suggestions noted for later, also try and accommodate if possible.

> thanks
> -- PMM
>