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Date: Wed, 12 May 2021 10:37:34 +0200
From: 'Joerg Roedel' <joro@8bytes.org>
To: David Laight <David.Laight@aculab.com>
Cc: "x86@kernel.org" <x86@kernel.org>, Hyunwook Baek <baekhw@google.com>,
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Subject: Re: [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user
Message-ID: <YJuTzhSp2XAJIYlv@8bytes.org>
References: <20210512075445.18935-1-joro@8bytes.org>
 <20210512075445.18935-4-joro@8bytes.org>
 <0496626f018d4d27a8034a4822170222@AcuMS.aculab.com>
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In-Reply-To: <0496626f018d4d27a8034a4822170222@AcuMS.aculab.com>

On Wed, May 12, 2021 at 08:04:33AM +0000, David Laight wrote:
> That can't be right at all.
> __put/get_user() are only valid on user addresses and will try to
> fault in a missing page - so can sleep.

Yes, in general these functions can sleep, but not in this context. They
are called in atomic context and the page-fault handler will notice that
and goes down the __bad_area_nosemaphore() path and only do the fixup.

I also thought about adding page_fault_disable()/page_fault_enable()
calls, but being in atomic context is enough according to the
faulthandler_disabled() implementation.

This is exactly what is needed here. All I want to know is whether a
fault happened or not, the page-fault handler must not try to fix the
fault in any way. If a fault happens it is later fixed up in
vc_forward_exception().

> At best this is abused the calls.

Yes, but that is only due to the naming of these functions. In this case
they do exactly what is needed.

Regards,

	Joerg