linux-kernel - Re: [RFC PATCH 03/13] x86/cpu: Enumerate User Interrupts support

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Date:   Fri, 24 Sep 2021 00:24:00 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org
Cc:     Sohil Mehta <sohil.mehta@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 03/13] x86/cpu: Enumerate User Interrupts support

On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote:
> SENDUIPI is a special ring-3 instruction that makes a supervisor mode
> memory access to the UPID and UITT memory. Currently, KPTI needs to be
> off for User IPIs to work.  Processors that support user interrupts are
> not affected by Meltdown so the auto mode of KPTI will default to off.
>
> Users who want to force enable KPTI will need to wait for a later
> version of this patch series that is compatible with KPTI. We need to
> allocate the UPID and UITT structures from a special memory region that
> has supervisor access but it is mapped into userspace. The plan is to
> implement a mechanism similar to LDT.

Seriously?

> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Signed-off-by: Sohil Mehta <sohil.mehta@...el.com>

This SOB chain is invalid. Ditto in several other patches.

>  
> +config X86_USER_INTERRUPTS
> +	bool "User Interrupts (UINTR)"
> +	depends on X86_LOCAL_APIC && X86_64

X86_64 does not work w/o LOCAL_APIC so this dependency is pointless.

> +	depends on CPU_SUP_INTEL
> +	help
> +	  User Interrupts are events that can be delivered directly to
> +	  userspace without a transition through the kernel. The interrupts
> +	  could be generated by another userspace application, kernel or a
> +	  device.
> +
> +	  Refer, Documentation/x86/user-interrupts.rst for details.

"Refer, Documentation..." is not a sentence.

>  
> +/* User Interrupt interface */
> +#define MSR_IA32_UINTR_RR		0x985
> +#define MSR_IA32_UINTR_HANDLER		0x986
> +#define MSR_IA32_UINTR_STACKADJUST	0x987
> +#define MSR_IA32_UINTR_MISC		0x988	/* 39:32-UINV, 31:0-UITTSZ */

Bah, these tail comments are crap. Please define proper masks/shift
constants for this instead of using magic numbers in the code.

> +static __always_inline void setup_uintr(struct cpuinfo_x86 *c)

This has to be always inline because it's performance critical or what?

> +{
> +	/* check the boot processor, plus compile options for UINTR. */

Sentences start with uppercase letters.

> +	if (!cpu_feature_enabled(X86_FEATURE_UINTR))
> +		goto disable_uintr;
> +
> +	/* checks the current processor's cpuid bits: */
> +	if (!cpu_has(c, X86_FEATURE_UINTR))
> +		goto disable_uintr;
> +
> +	/*
> +	 * User Interrupts currently doesn't support PTI. For processors that
> +	 * support User interrupts PTI in auto mode will default to off.  Need
> +	 * this check only for users who have force enabled PTI.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_PTI)) {
> +		pr_info_once("x86: User Interrupts (UINTR) not enabled. Please disable PTI using 'nopti' kernel parameter\n");

That message does not make sense. The admin has explicitly added 'pti'
to the kernel command line on a CPU which is not affected. So why would
he now have to add 'nopti' ?

Thanks,

        tglx