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Subject: Re: [RFC PATCH 03/13] x86/cpu: Enumerate User Interrupts support
To:     Thomas Gleixner <tglx@linutronix.de>, <x86@kernel.org>
CC:     Tony Luck <tony.luck@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        Christian Brauner <christian@brauner.io>,
        Peter Zijlstra <peterz@infradead.org>,
        Shuah Khan <shuah@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Ashok Raj <ashok.raj@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        "Gayatri Kammela" <gayatri.kammela@intel.com>,
        Zeng Guang <guang.zeng@intel.com>,
        "Dan Williams" <dan.j.williams@intel.com>,
        Randy E Witt <randy.e.witt@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Ramesh Thomas <ramesh.thomas@intel.com>,
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        <linux-kernel@vger.kernel.org>, <linux-kselftest@vger.kernel.org>
References: <20210913200132.3396598-1-sohil.mehta@intel.com>
 <20210913200132.3396598-4-sohil.mehta@intel.com> <87lf3nexrz.ffs@tglx>
From:   Sohil Mehta <sohil.mehta@intel.com>
Message-ID: <42a14757-5b38-19cf-d830-1641b07f89ba@intel.com>
Date:   Mon, 27 Sep 2021 13:42:33 -0700
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On 9/23/2021 3:24 PM, Thomas Gleixner wrote:
> 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?

Are questioning why we should add KPTI support if the hardware is not 
affected by Meltdown?

or

Why use an LDT like mechanism to do this?

I have listed this as one of the opens in the cover letter as well. I am 
not sure if users who force enable PTI would really care about User 
Interrupts.

Any input here would be helpful.

>
>> +	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' ?

Yup. I'll fix this and other issues in this patch.

I thought the user should know why UINTR has been disabled. In 
hindsight, this would have been better covered in the sample Readme or 
something similar.


Thanks,

Sohil