linux-kernel - Re: [PATCH] arm64: Independently update HDFGRTR_EL2 and HDFGWTR

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Date:   Thu, 19 Oct 2023 14:01:31 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: Independently update HDFGRTR_EL2 and HDFGWTR_EL2



On 10/19/23 12:45, Marc Zyngier wrote:
> On Thu, 19 Oct 2023 04:36:15 +0100,
> Anshuman Khandual <anshuman.khandual@....com> wrote:
>>
>>
>>
>> On 10/18/23 18:10, Marc Zyngier wrote:
>>> On Wed, 18 Oct 2023 04:00:07 +0100,
>>> Anshuman Khandual <anshuman.khandual@....com> wrote:
>>>>
>>>> Currently PMSNEVFR_EL1 system register read, and write access EL2 traps are
>>>> disabled, via setting the same bit (i.e 62) in HDFGRTR_EL2, and HDFGWTR_EL2
>>>> respectively. Although very similar, bit fields are not exact same in these
>>>> two EL2 trap configure registers particularly when it comes to read-only or
>>>> write-only accesses such as ready-only 'HDFGRTR_EL2.nBRBIDR' which needs to
>>>> be set while enabling BRBE on NVHE platforms. Using the exact same bit mask
>>>> fields for both these trap register risk writing into their RESERVED areas,
>>>> which is undesirable.
>>>
>>> Sorry, I don't understand at all what you are describing. You seem to
>>> imply that the read and write effects of the FGT doesn't apply the
>>> same way. But my reading of the ARM ARM is that  behave completely
>>> symmetrically.
>>>
>>> Also, what is nBRBIDR doing here? It is still set to 0. What
>>> 'RESERVED' state are you talking about?
>>
>> Let's observe the following example which includes the nBRBIDR problem,
>> mentioned earlier.
>>
>> Read access trap configure
>>
>> HDFGRTR_EL2[59]	   - nBRBIDR
>> HDFGRTR_EL2[58]	   - PMCEIDn_EL0
>>
>> Write access trap configure
>>
>> HDFGWTR_EL2[59:58] - RES0
>>
>> Because BRBIDR_EL1 and PMCEID<N>_EL0 are read only registers they don't
>> have corresponding entries in HDFGWTR_EL2 for write trap configuration.
>>
>> Using the exact same value contained in 'x0' both for HDFGRTR_EL2, and
>> HDFGWTR_EL2 will be problematic in case it contains bit fields that are
>> available only in one of the registers but not in the other.
>>
>> If 'x0' contains nBRBIDR being set, it will be okay for HDFGRTR_EL2 but
>> might not be okay for HDFGWTR_EL2 where it will get into RESERVED areas.
> 
> None of which matters for this patch. You keep arguing about something
> that does not exist in the change you're proposing.
> 
> [...]
> 
>> I should have given more details in the commit message but hope
>> you have some context now, but please do let me know if there
>> is something still missing.
> 
> What is missing is a useful patch. This one just obfuscates things for
> no particular purpose. If you have a useful change to contribute,
> please send that instead (your BRBE change). We don't need an extra,
> standalone and pointless patch such as this one.

I will fold this patch with other BRBE changes as mentioned earlier but
thought that - separating out updates for HDFGRTR_EL2, and HDFGWTR_EL2
should be done as stand alone change in a preparatory patch. Seems like
that was an incorrect assumption.