linux-kernel - Re: [PATCH v3 0/6] arm64: add the time namespace support

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Message-ID: <08e3ee73-8048-b84e-61b3-e87f3500e022@arm.com>
Date:   Wed, 20 May 2020 13:02:26 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Andrei Vagin <avagin@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH v3 0/6] arm64: add the time namespace support

Hi Andrei,

On 4/16/20 6:26 AM, Andrei Vagin wrote:
> Allocate the time namespace page among VVAR pages and add the logic
> to handle faults on VVAR properly.
> 
> If a task belongs to a time namespace then the VVAR page which contains
> the system wide VDSO data is replaced with a namespace specific page
> which has the same layout as the VVAR page. That page has vdso_data->seq
> set to 1 to enforce the slow path and vdso_data->clock_mode set to
> VCLOCK_TIMENS to enforce the time namespace handling path.
> 
> The extra check in the case that vdso_data->seq is odd, e.g. a concurrent
> update of the VDSO data is in progress, is not really affecting regular
> tasks which are not part of a time namespace as the task is spin waiting
> for the update to finish and vdso_data->seq to become even again.
> 
> If a time namespace task hits that code path, it invokes the corresponding
> time getter function which retrieves the real VVAR page, reads host time
> and then adds the offset for the requested clock which is stored in the
> special VVAR page.
> 
> v2: Code cleanups suggested by Vincenzo.
> v3: use OPTIMIZER_HIDE_VAR() instead of inline assembly in
>     __arch_get_timens_vdso_data.
> 

Nit: If you re-post, I would remove the OPTIMIZER_HIDE_VAR() reference because
it does not reflect the current status of the patches.

I tested it again with your latest change in the test code and it works for me
(thank you for sending a patch for the test as well).

With this:

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>

> Cc: Vincenzo Frascino <vincenzo.frascino@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Dmitry Safonov <dima@...sta.com>
> 
> v3 on github (if someone prefers `git pull` to `git am`):
> https://github.com/avagin/linux-task-diag/tree/arm64/timens-v3
> 
> Andrei Vagin (6):
>   arm64/vdso: use the fault callback to map vvar pages
>   arm64/vdso: Zap vvar pages when switching to a time namespace
>   arm64/vdso: Add time napespace page
>   arm64/vdso: Handle faults on timens page
>   arm64/vdso: Restrict splitting VVAR VMA
>   arm64: enable time namespace support
> 
>  arch/arm64/Kconfig                            |   1 +
>  .../include/asm/vdso/compat_gettimeofday.h    |  11 ++
>  arch/arm64/include/asm/vdso/gettimeofday.h    |   8 ++
>  arch/arm64/kernel/vdso.c                      | 134 ++++++++++++++++--
>  arch/arm64/kernel/vdso/vdso.lds.S             |   3 +-
>  arch/arm64/kernel/vdso32/vdso.lds.S           |   3 +-
>  include/vdso/datapage.h                       |   1 +
>  7 files changed, 147 insertions(+), 14 deletions(-)
> 

-- 
Regards,
Vincenzo