Re: [RFC PATCH 04/13] x86/fpu/xstate: Enumerate User Interrupts supervisor state

From: Sohil Mehta <sohil.mehta@intel.com>
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>,
	<linux-api@vger.kernel.org>, <linux-arch@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <linux-kselftest@vger.kernel.org>
Subject: Re: [RFC PATCH 04/13] x86/fpu/xstate: Enumerate User Interrupts supervisor state
Date: Mon, 27 Sep 2021 15:25:34 -0700	[thread overview]
Message-ID: <c364342a-833d-0c68-c496-97a843e512a5@intel.com> (raw)
In-Reply-To: <87ilyqgbvn.ffs@tglx>

On 9/23/2021 3:34 PM, Thomas Gleixner wrote:
> On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote:
>> Enable xstate supervisor support for User Interrupts by default.
> What means enabled by default? It's enabled when available and not
> disabled on the command line.

I'll remove it.

>> The user interrupt state for a task consists of the MSR state and the
>> User Interrupt Flag (UIF) value. XSAVES and XRSTORS handle saving and
>> restoring both of these states.
>>
>> <The supervisor XSTATE code might be reworked based on issues reported
>> in the past. The Uintr context switching code would also need rework and
>> additional testing in that regard.>
> What? Which issues were reported and if they have been reported then how
> is the provided code correct?

I apologize for causing this confusion. This comment was added a few 
months back when the PKRU and FPU code was being reworked. This comment 
is no longer valid.

>> +/*
>> + * State component 14 is supervisor state used for User Interrupts state.
>> + * The size of this state is 48 bytes
>> + */
>> +struct uintr_state {
>> +	u64 handler;
>> +	u64 stack_adjust;
>> +	u32 uitt_size;
>> +	u8  uinv;
>> +	u8  pad1;
>> +	u8  pad2;
>> +	u8  uif_pad3;		/* bit 7 - UIF, bits 6:0 - reserved */
> Please do not use tail comments. Also what kind of name is uif_pad3?
> Bitfields exist for a reason.

An internal version of this used bitfields. I was suggested that use of 
bitfields is not recommended for x86 code.

The name uif_pad3 is really ugly though. I'll change it to a bitfield 
next time.

Thanks,

Sohil