linux-kernel - Re: kernel 3.7+ cpufreq regression on AMD system running as dom0

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Date:	Mon, 21 Jan 2013 13:42:55 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Stefan Bader <stefan.bader@...onical.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andre Przywara <andre@...rep.de>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Matthew Garrett <mjg@...hat.com>
Subject: Re: kernel 3.7+ cpufreq regression on AMD system running as dom0

On Mon, Jan 21, 2013 at 12:22:18PM +0000, Stefan Bader wrote:
> So for having the "check for sensible BIOS" in mainline I refreshed
> the patch (fixed the bit test, and actually tested it this time) and
> also added some hopefully sensible explanation to it (attached
> below).
> 
> Should I send it to acpi lists or would that have to go via an Andre?

Maybe Rafael could pick it up?

> 
> -Stefan
> 
> From 6e2fc8291c91339123a37162382d8b08b50867ba Mon Sep 17 00:00:00 2001
> From: Stefan Bader <stefan.bader@...onical.com>
> Date: Mon, 14 Jan 2013 16:17:00 +0100
> Subject: [PATCH] ACPI: Check MSR valid bit before using P-state frequencies
> 
> To fix incorrect P-state frequencies which can happen on
> some AMD systems f594065faf4f9067c2283a34619fc0714e79a98d
>   "ACPI: Add fixups for AMD P-state figures"
> introduced a quirk to obtain the correct values by reading
> from AMD specific MSRs.
> 
> This did cause a regression when running a kernel using that
> quirk under Xen which does (currently) not pass on the contents
> of the HW but 0.

Actually this should say "does not currently pass through MSR accesses
to baremetal" or similar.

And this bit you mean is actually bit 63:

"63: PstateEn. Read-write. 1=The P-state specified by this MSR is valid.
0=The P-state specified by this MSR is not valid. The purpose of this
register is to indicate if the rest of the P-state information in the
register is valid after a reset; it controls no hardware."

in the MSRC001_00[68:64] P-State [4:0] Registers.

> And this seems to cause a failure to initialize
> the ondemand governour (hard to say for sure as all P-states
> appear to run at the same frequency).
> 
> While this should also be fixed in the hypervisor (to allow
> a guest to read that MSR), this patch is intended to work
> around the issue in the meantime. In discussion it turned out
> that indeed real HW/BIOSes may choose to not set the valid bit
> and thus mark the P-state as invalid. So this could be considered
> a fix for broken BIOSes that also works around the issue on Xen.
> 
> Signed-off-by: Stefan Bader <stefan.bader@...onical.com>
> Cc: stable@...r.kernel.org # v3.7..
> ---
>  drivers/acpi/processor_perflib.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/acpi/processor_perflib.c b/drivers/acpi/processor_perflib.c
> index 836bfe0..41f4bdac 100644
> --- a/drivers/acpi/processor_perflib.c
> +++ b/drivers/acpi/processor_perflib.c
> @@ -340,6 +340,9 @@ static void amd_fixup_frequency(struct
> acpi_processor_px *px, int i)
>  	if ((boot_cpu_data.x86 == 0x10 && boot_cpu_data.x86_model < 10)
>  	    || boot_cpu_data.x86 == 0x11) {
>  		rdmsr(MSR_AMD_PSTATE_DEF_BASE + index, lo, hi);
> +		/* Bit 63 indicates whether contents are valid */
> +		if (!(hi & 0x80000000))

You can make this a lot more explicit:

		if (!(hi & BIT(31)))
			return;

This way

1) you're sure you're testing the correct bit and
2) any reviewer can know on the spot which bit it is about.

> +			return;
>  		fid = lo & 0x3f;
>  		did = (lo >> 6) & 7;
>  		if (boot_cpu_data.x86 == 0x10)

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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