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From: James Morse <james.morse@arm.com>
To: Julien Grall <julien.grall@arm.com>
Cc: linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, marc.zyngier@arm.com,
	julien.thierry@arm.com, suzuki.poulose@arm.com,
	catalin.marinas@arm.com, will.deacon@arm.com
Subject: Re: [RFC v2 12/14] arm64/lib: asid: Allow user to update the context under the lock
Date: Wed, 3 Jul 2019 18:35:23 +0100	[thread overview]
Message-ID: <c5d1257c-b522-152f-cb2f-d23fd8110609@arm.com> (raw)
In-Reply-To: <20190620130608.17230-13-julien.grall@arm.com>

Hi Julien,

On 20/06/2019 14:06, Julien Grall wrote:
> Some users of the ASID allocator (e.g VMID) will require to update the
> context when a new ASID is generated. This has to be protected by a lock
> to prevent concurrent modification.
> 
> Rather than introducing yet another lock, it is possible to re-use the
> allocator lock for that purpose. This patch introduces a new callback
> that will be call when updating the context.

You're using this later in the series to mask out the generation from the atomic64 to
leave just the vmid.

Where does this concurrent modification happen? The value is only written if we have a
rollover, and while its active the only bits that could change are the generation.
(subsequent vCPUs that take the slow path for the same VM will see the updated generation
and skip the new_context call)

If we did the generation filtering in update_vmid() after the call to
asid_check_context(), what would go wrong?
It happens more often than is necessary and would need a WRITE_ONCE(), but the vmid can't
change until we become preemptible and another vCPU gets a chance to make its vmid active.

This thing is horribly subtle, so I'm sure I've missed something here!

I can't see where the arch code's equivalent case is. It also filters the generation out
of the atomic64 in cpu_do_switch_mm(). This happens with interrupts masked so we can't
re-schedule and set another asid as 'active'. KVM's equivalent is !preemptible.


Thanks,

James

  reply	other threads:[~2019-07-03 17:35 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-20 13:05 [RFC v2 00/14] kvm/arm: Align the VMID allocation with the arm64 ASID one Julien Grall
2019-06-20 13:05 ` [RFC v2 01/14] arm64/mm: Introduce asid_info structure and move asid_generation/asid_map to it Julien Grall
2019-06-20 13:05 ` [RFC v2 02/14] arm64/mm: Move active_asids and reserved_asids to asid_info Julien Grall
2019-06-20 13:05 ` [RFC v2 03/14] arm64/mm: Move bits " Julien Grall
2019-06-20 13:05 ` [RFC v2 04/14] arm64/mm: Move the variable lock and tlb_flush_pending " Julien Grall
2019-06-20 13:05 ` [RFC v2 05/14] arm64/mm: Remove dependency on MM in new_context Julien Grall
2019-06-20 13:06 ` [RFC v2 06/14] arm64/mm: Store the number of asid allocated per context Julien Grall
2019-06-20 13:06 ` [RFC v2 07/14] arm64/mm: Introduce NUM_ASIDS Julien Grall
2019-06-20 13:06 ` [RFC v2 08/14] arm64/mm: Split asid_inits in 2 parts Julien Grall
2019-06-20 13:06 ` [RFC v2 09/14] arm64/mm: Split the function check_and_switch_context in 3 parts Julien Grall
2019-06-20 13:06 ` [RFC v2 10/14] arm64/mm: Introduce a callback to flush the local context Julien Grall
2019-06-20 13:06 ` [RFC v2 11/14] arm64: Move the ASID allocator code in a separate file Julien Grall
2019-07-04 14:56   ` James Morse
2019-07-15 10:58     ` Julien Grall
2019-06-20 13:06 ` [RFC v2 12/14] arm64/lib: asid: Allow user to update the context under the lock Julien Grall
2019-07-03 17:35   ` James Morse [this message]
2019-07-15 14:38     ` Julien Grall
2019-06-20 13:06 ` [RFC v2 13/14] arm/kvm: Introduce a new VMID allocator Julien Grall
2019-06-20 13:06 ` [RFC v2 14/14] kvm/arm: Align the VMID allocation with the arm64 ASID one Julien Grall
2019-07-03 17:36   ` James Morse
2019-07-15 17:06     ` Julien Grall

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