Re: [PATCH v4 3/3] arm64: Add workaround for Arm Cortex-A77 erratum 1508412

From: Marc Zyngier <maz@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Rob Herring <robh@kernel.org>, Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 3/3] arm64: Add workaround for Arm Cortex-A77 erratum 1508412
Date: Fri, 21 Aug 2020 13:45:40 +0100	[thread overview]
Message-ID: <8c641833ff20d5a35981c456d4fe1d5a@kernel.org> (raw)
In-Reply-To: <20200821122633.GC6823@gaia>

On 2020-08-21 13:26, Catalin Marinas wrote:
> On Fri, Aug 21, 2020 at 01:12:10PM +0100, Will Deacon wrote:
>> On Fri, Aug 21, 2020 at 01:07:00PM +0100, Catalin Marinas wrote:
>> > On Mon, Aug 03, 2020 at 01:31:27PM -0600, Rob Herring wrote:
>> > > @@ -979,6 +980,14 @@
>> > >  		write_sysreg(__scs_new, sysreg);			\
>> > >  } while (0)
>> > >
>> > > +#define read_sysreg_par() ({						\
>> > > +	u64 par;							\
>> > > +	asm(ALTERNATIVE("nop", "dmb sy", ARM64_WORKAROUND_1508412));	\
>> > > +	par = read_sysreg(par_el1);					\
>> > > +	asm(ALTERNATIVE("nop", "dmb sy", ARM64_WORKAROUND_1508412));	\
>> > > +	par;								\
>> > > +})
>> >
>> > I was about to queue this up but one more point to clarify: can we get
>> > an interrupt at either side of the PAR_EL1 read and the handler do a
>> > device read, triggering the erratum? Do we need a DMB at exception
>> > entry/return?
>> 
>> Disabling irqs around the PAR access would be simpler, I think 
>> (assuming
>> this is needed).
> 
> This wouldn't work if it interrupts a guest.

If we take an interrupt either side of the PAR_EL1 read and that we
fully exit, the saving of PAR_EL1 on the way out solves the problem.

If we don't fully exit, but instead reenter the guest immediately
(fixup_guest_exit() returns true), we'd need a DMB at that point,
at least because of the GICv2 proxying code which performs device
accesses on the guest's behalf.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...
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