linux-kernel - Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on demand

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Message-ID: <20230627121853.ek5zr7sfiezfkfyj@box.shutemov.name>
Date:   Tue, 27 Jun 2023 15:18:53 +0300
From:   "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on
 demand

On Tue, Jun 27, 2023 at 10:34:04AM +0000, Huang, Kai wrote:
> On Tue, 2023-06-27 at 12:50 +0300, kirill.shutemov@...ux.intel.com wrote:
> > On Tue, Jun 27, 2023 at 02:12:37AM +1200, Kai Huang wrote:
> > > +	/*
> > > +	 * The TDX module global initialization only needs to be done
> > > +	 * once on any cpu.
> > > +	 */
> > > +	raw_spin_lock_irqsave(&tdx_global_init_lock, flags);
> > 
> > I don't understand how the comment justifies using raw spin lock.
> > 
> 
> This comment is for using lock in general.  The reason to use raw_ version is
> because this function gets called in IRQ context, and for PREEMPT_RT kernel the
> normal spinlock is converted to sleeping lock.

Sorry, but this still doesn't explain anything.

Why converting to sleeping lock here is wrong? There are plenty
spin_lock_irqsave() users all over the kernel that are fine to be
converted to sleeping lock on RT kernel. Why this use-case is special
enough to justify raw_?

>From the documentation:

	raw_spinlock_t is a strict spinning lock implementation in all
	kernels, including PREEMPT_RT kernels. Use raw_spinlock_t only in
	real critical core code, low-level interrupt handling and places
	where disabling preemption or interrupts is required, for example,
	to safely access hardware state. raw_spinlock_t can sometimes also
	be used when the critical section is tiny, thus avoiding RT-mutex
	overhead.

How does it apply here?

> Dave suggested to comment on the function rather than comment on the
> raw_spin_lock directly, e.g.,  no other kernel code does that:
> 
> https://lore.kernel.org/linux-mm/d2b3bc5e-1371-0c50-8ecb-64fc70917d42@intel.com/
> 
> So I commented the function in this version:
> 
> +/*
> + * Do the module global initialization if not done yet.
> + * It's always called with interrupts and preemption disabled.
> + */

If interrupts are always disabled why do you need _irqsave()?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov