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

From: Suzuki K. Poulose
Date: Mon Nov 23 2015 - 18:48:20 EST

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.

+ 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.


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