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

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Message-ID: <ZTA9BlmagUv1teh7@linux.dev>
Date:   Wed, 18 Oct 2023 20:16:06 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: Independently update HDFGRTR_EL2 and HDFGWTR_EL2

On Wed, Oct 18, 2023 at 01:40:37PM +0100, 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.

nBRBIDR is an asymmetric bit (bit 59 of HDFGWTR_EL2 is RES0). While the
architecture *could* repurpose this WTR bit for something else, that
feels rather implementation and software hostile. I don't think there's
a practical issue here, especially since the architecture has already
allocated another pair of debug trap registers to make room for more
bits.

> So what has changed here, aside from clobbering an extra register? The
> masks are the same, the initial values are the same... Is it in
> preparation for some other work?

Yeah, it feels as though this patch is taken out of context. Without a
justifying functional change I don't see the value in fiddling with this
code.

-- 
Thanks,
Oliver