From: Julien Grall <julien.grall@arm.com>
To: James Morse <james.morse@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: Mon, 15 Jul 2019 15:38:42 +0100 [thread overview]
Message-ID: <446cfa1a-71be-3ae2-4107-02dd0f164843@arm.com> (raw)
In-Reply-To: <c5d1257c-b522-152f-cb2f-d23fd8110609@arm.com>
On 03/07/2019 18:35, James Morse wrote:
> Hi Julien,
Hi James,
> 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.
You are right.
>
> 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.
I think I was over cautious. Pre-filtering after asid_check_context() is equally
fine as long as update_vttbr() is called from preemptible context.
Cheers,
--
Julien Grall
next prev parent reply other threads:[~2019-07-15 14:38 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
2019-07-15 14:38 ` Julien Grall [this message]
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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