netdev - RE: [PATCH v7 04/21] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access

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Message-ID: <c8ca959a4bc549b59baca563660d42d6@AcuMS.aculab.com>
Date:   Thu, 18 Apr 2019 09:20:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Fenghua Yu' <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        "Xiaoyao Li " <xiaoyao.li@...el.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Michael Chan <michael.chan@...adcom.com>
CC:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
Subject: RE: [PATCH v7 04/21] x86/split_lock: Align x86_capability to unsigned
 long to avoid split locked access

From: Fenghua Yu
> Sent: 17 April 2019 22:34
> 
> set_cpu_cap() calls locked BTS and clear_cpu_cap() calls locked BTR to
> operate on bitmap defined in x86_capability.
> 
> Locked BTS/BTR accesses a single unsigned long location. In 64-bit mode,
> the location is at:
> base address of x86_capability + (bit offset in x86_capability / 64) * 8
> 
> Since base address of x86_capability may not be aligned to unsigned long,
> the single unsigned long location may cross two cache lines and
> accessing the location by locked BTS/BTR introductions will cause
> split lock.

Isn't the problem that the type (and definition) of x86_capability[] are wrong.
If the 'bitmap' functions are used for it, it should be defined as a bitmap.
This would make it 'unsigned long' not __u32.

This type munging of bitmaps only works on LE systems.

OTOH the locked BTS/BTR instructions could be changed to use 32 bit accesses.
ISTR some of the associated functions use byte accesses.

Perhaps there ought to be asm wrappers for BTS/BTR that do 8bit and
32bit accesses.

> 
> To fix the split lock issue, align x86_capability to size of unsigned long
> so that the location will be always within one cache line.
> 
> Changing x86_capability's type to unsigned long may also fix the issue
> because x86_capability will be naturally aligned to size of unsigned long.
> But this needs additional code changes. So choose the simpler solution
> by setting the array's alignment to size of unsigned long.
> 
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> ---
>  arch/x86/include/asm/processor.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 2bb3a648fc12..7c62b9ad6e5a 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -93,7 +93,9 @@ struct cpuinfo_x86 {
>  	__u32			extended_cpuid_level;
>  	/* Maximum supported CPUID level, -1=no CPUID: */
>  	int			cpuid_level;
> -	__u32			x86_capability[NCAPINTS + NBUGINTS];
> +	/* Aligned to size of unsigned long to avoid split lock in atomic ops */
> +	__u32			x86_capability[NCAPINTS + NBUGINTS]
> +				__aligned(sizeof(unsigned long));
>  	char			x86_vendor_id[16];
>  	char			x86_model_id[64];
>  	/* in KB - valid for CPUS which support this call: */
> --
> 2.19.1

-
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