linux-kernel - Re: [PATCH 5/5] arm64: Ensure the secondary CPUs have safe ASIDBits size

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Date:	Mon, 23 Nov 2015 23:48:12 +0000
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
	mark.rutland@....com, ard.biesheuvel@...aro.org,
	linux-kernel@...r.kernel.org, takahiro.akashi@...aro.org
Subject: Re: [PATCH 5/5] arm64: Ensure the secondary CPUs have safe ASIDBits
 size

On 23/11/15 17:29, Will Deacon wrote:
> On Wed, Nov 18, 2015 at 05:09:00PM +0000, Suzuki K. Poulose wrote:
>> The ID_AA64MMFR0_EL1:ASIDBits determines the size of the mm context
>> id and is used in the early boot to make decisions. The value is
>> picked up from the Boot CPU and cannot be delayed until other CPUs
>> are up. If a secondary CPU has a smaller size than that of the Boot
>> CPU, things will break horribly and the usual SANITY check is not good
>> enough to prevent the system from crashing. Prevent this by failing CPUs with
>> ASID smaller than that of the boot CPU.

...

>> +	pr_crit("CPU%d: will not boot\n", cpu);
>
> This is less informative than the current message (whcih describes the
> missing capability).

The missing capability is printed by the caller (Patch 4/5) and see below
for the new user.


	ID_AA64MMFR0_ASID_SHIFT);
>> +	if (asid_cur < asid_boot) {
>> +		pr_crit("CPU%d: has incompatible ASIDBits: %u vs Boot CPU:%u\n",
>> +				cpu, asid_cur, asid_boot);
>> +		fail_incapable_cpu();
>> +	}
>
> Hmm. Whilst we want to ensure that secondary CPUs don't have a smaller
> ASID size than the boot CPU, can we actually guarantee that a smaller
> value for ID_AA64MMFR0.ASIDBits corresponds to fewer bits? We're
> probably better off assuming 8-bit ASIDs unless ASIDBits == 2 (which is
> what the ASID allocator does).

Right, I already have a reworked version, which does the same and moves it
back to verify_local_cpu_capabilities(). I will send it tomorrow.

Cheers
Suzuki

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