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

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Message-ID: <8734y76q2w.wl-maz@kernel.org>
Date:   Thu, 19 Oct 2023 08:15:03 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
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 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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.