linux-kernel - Re: [PATCH] x86/sgx: Replace kmap/kunmap

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Date:   Thu, 6 Oct 2022 14:26:41 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        <linux-kernel@...r.kernel.org>, <linux-sgx@...r.kernel.org>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Borislav Petkov" <bp@...en8.de>, <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>
Subject: Re: [PATCH] x86/sgx: Replace kmap/kunmap_atomic calls

On Thu, Oct 06, 2022 at 01:45:56PM -0700, Hansen, Dave wrote:
> On 10/6/22 13:37, Fabio M. De Francesco wrote:
> > kmap() were not suited in those cases because it might sleep. If the intents 
> > of the author are simply map a page while in atomic, so to avoid sleeping in 
> > atomic bugs, your conversions looks good. 
> > 
> > For the reasons above, can you please say something more about why this code 
> > needed a kmap_atomic() instead of calling kmap()?
> 
> This question is backwards.  kmap_atomic() is the default that folks
> use.  You use kmap_atomic() *always* unless you _need_ to sleep or one
> of the other kmap()-only things.
> 
> Folks don't and shouldn't have to explain why this was using kmap_atomic().

I've not looked at the code closely enough but there was some concern that
kmap_atomic() callers are depending on preemption being disabled vs the more
common case of them being used in an atomic context where kmap() can't work.

Ira