linux-kernel - Re: [PATCH v3 2/3] x86/platform/uv: Update TSC sync state for UV5

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Date:   Tue, 05 Apr 2022 02:47:04 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mike Travis <mike.travis@....com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>,
        Steve Wahl <steve.wahl@....com>, x86@...nel.org
Cc:     Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Andy Shevchenko <andy@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Russ Anderson <russ.anderson@....com>,
        linux-kernel@...r.kernel.org, platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v3 2/3] x86/platform/uv: Update TSC sync state for UV5

Mike,

On Mon, Apr 04 2022 at 12:41, Mike Travis wrote:
> Update to not check TSC sync state for uv5+ as it is not available.
> It is assumed that TSC will always be in sync for multiple chassis and
> will pass the tests for the kernel to accept it as the clocksource.
> To disable this check use the kernel start options tsc=reliable
> clocksource=tsc.
...
> ---
> v2: Update patch description to be more explanatory.

after reading the above word salad, I have no urge to go back to V1 to
figure out how bad that changelog was. Let me walk through it:

> Update to not check TSC sync state for uv5+ as it is not available.

That's not a sentence and it does not provide any information about the
background and the why. See:

  https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog

for further explanation.

> It is assumed that TSC will always be in sync for multiple chassis and
> will pass the tests for the kernel to accept it as the clocksource.

"It is assumed" is a real convincing technical argument - NOT!

Either the hardware guarantees something or not. If the hardware cannot
guarantee it but does not provide a mechanism to verify it then spell it
out:

  "UV5 gave up on providing an interface which allows to query the TSC
   synchronization state. The hardware/firmware specification defines
   that TSC is synchronized accross multiple chassis, but there is
   neither a guarantee nor a validation mechanism. This leaves the
   kernel with the only option to assume that TSC is synchronized and
   TSC_ADJUST might be different between sockets due to UV5+ firmware
   synchronization."

> To disable this check use the kernel start options tsc=reliable
> clocksource=tsc.

So now it get's really confusing. Which check? The one in your 
explanatory description above:

  "Update to not check TSC sync state for uv5+ as it is not available."

or what?

I can assume what are you trying to tell me here, but that's not a
qualification for 'explanatory'

> Signed-off-by: Mike Travis <mike.travis@....com>
> Reviewed-by: Dimitri Sivanich <dimitri.sivanich@....com>
> Reviewed-by: Steve Wahl <steve.wahl@....com>

Impressive list of Reviewed-by tags. Just for clarification:

  Reviewed-by tags include the changelog part.

> ---
>  arch/x86/kernel/apic/x2apic_uv_x.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/apic/x2apic_uv_x.c b/arch/x86/kernel/apic/x2apic_uv_x.c
> index f5a48e66e4f5..387d6533549a 100644
> --- a/arch/x86/kernel/apic/x2apic_uv_x.c
> +++ b/arch/x86/kernel/apic/x2apic_uv_x.c
> @@ -199,10 +199,16 @@ static void __init uv_tsc_check_sync(void)
>  	int mmr_shift;
>  	char *state;
>  
> -	/* Different returns from different UV BIOS versions */
> +	/* UV5+, sync state from bios not available, assumed valid */
> +	if (!is_uv(UV2|UV3|UV4)) {
> +		pr_debug("UV: TSC sync state for UV5+ assumed valid\n");

UV advertises its version all over the place and the call here:

> +		mark_tsc_async_resets("UV5+");

emits the reason at info level. So you surely need the pr_debug() above
to figure out that your code works as expected. You just failed to add
the obviuos counterpart:

          pr_debug("After calling hello_world()!\n");

Seriously. Neither the changelog nor the comment above the condition
qualify for explanatory or useful. Please try again.

> +
> +	/* UV2,3,4, UV BIOS TSC sync state available */
>  	mmr = uv_early_read_mmr(UVH_TSC_SYNC_MMR);
> -	mmr_shift =
> -		is_uv2_hub() ? UVH_TSC_SYNC_SHIFT_UV2K : UVH_TSC_SYNC_SHIFT;
> +	mmr_shift = is_uv2_hub() ? UVH_TSC_SYNC_SHIFT_UV2K : UVH_TSC_SYNC_SHIFT;

How is this change related to the problem which this patch tries to
solve? Documentation/process/* applies to UV too. You definitely should
know that by now.

Thanks,

        tglx