Re: [PATCH] arm64: clear_page() shouldn't use DC ZVA when DCZID_EL0.DZP == 1

From: Will Deacon <will@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Reiji Watanabe <reijiw@google.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	Peter Shier <pshier@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Oliver Upton <oupton@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>
Subject: Re: [PATCH] arm64: clear_page() shouldn't use DC ZVA when DCZID_EL0.DZP == 1
Date: Thu, 28 Oct 2021 08:46:10 +0100	[thread overview]
Message-ID: <20211028074609.GA23476@willie-the-truck> (raw)
In-Reply-To: <20211027110947.GB51402@C02TD0UTHF1T.local>

On Wed, Oct 27, 2021 at 12:09:47PM +0100, Mark Rutland wrote:
> On Tue, Oct 26, 2021 at 11:44:51PM -0700, Reiji Watanabe wrote:
> > On Tue, Oct 26, 2021 at 5:23 AM Mark Rutland <mark.rutland@arm.com> wrote:
> > > On Tue, Oct 26, 2021 at 12:22:20PM +0100, Robin Murphy wrote:
> > > > On 2021-10-26 04:48, Reiji Watanabe wrote:
> > > > > Currently, clear_page() uses DC ZVA instruction unconditionally.  But it
> > > > > should make sure that DCZID_EL0.DZP, which indicates whether or not use
> > > > > of DC ZVA instruction is prohibited, is zero when using the instruction.
> > > > > Use stp as memset does instead when DCZID_EL0.DZP == 1.
> > > > >
> > > > > Signed-off-by: Reiji Watanabe <reijiw@google.com>
> > > > > ---
> > > > >   arch/arm64/lib/clear_page.S | 11 +++++++++++
> > > > >   1 file changed, 11 insertions(+)
> > > > >
> > > > > diff --git a/arch/arm64/lib/clear_page.S b/arch/arm64/lib/clear_page.S
> > > > > index b84b179edba3..7ce1bfa4081c 100644
> > > > > --- a/arch/arm64/lib/clear_page.S
> > > > > +++ b/arch/arm64/lib/clear_page.S
> > > > > @@ -16,6 +16,7 @@
> > > > >    */
> > > > >   SYM_FUNC_START_PI(clear_page)
> > > > >     mrs     x1, dczid_el0
> > > > > +   tbnz    x1, #4, 2f      /* Branch if DC GVA is prohibited */
> > >
> > > DCZID_EL0.DZP (AKA DCZID_EL0[4]) says whether all of DC {ZVA,GVA,GZVA}
> > > are prohibited. This loop uses DZ ZVA, not GC GVA, so it'd be nice to
> > > s/GVA/ZVA/ here.
> > 
> > Thank you for catching it ! I will fix that.
> > 
> > > Howver, `DC GVA` and `DC GZVA` are both used in mte_set_mem_tag_range(),
> > > which'll need a similar update...
> > 
> > Yes, I'm aware of that and mte_zero_clear_page_tags() needs to get
> > updated as well.  But, Since I'm not familiar with MTE (and I don't
> > have any plans to use MTE yet), I didn't work on them (I'm not sure
> > how I can test them).
> > I might try to fix them separately later as well when I have time
> > (not so soon most likely though).
> 
> My view is that we should either:
> 
> * Document that we require DCZID_EL0.DZP==0, as is implicitly the case
>   today.

I disagree with that. There's nothing wrong in trapping this stuff, as long
as you go ahead and emulate it, which is exactly why we aren't checking it at
the moment as EL2 should be prepared to handle the trap. The Arm ARM talks
about the instructions being "prohibited" but that doesn't mean anything --
the reality is that they trap to EL2.

We could document *that* though?

Will

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