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Subject: Re: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state
To:     Rik van Riel <riel@surriel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>
References: <20180912133353.20595-1-bigeasy@linutronix.de>
 <20180912133353.20595-5-bigeasy@linutronix.de>
 <8e5b64e4-b3e6-f884-beb6-b7b69ab2d8c1@redhat.com>
 <20180914203501.qibhpmueosvkr74w@linutronix.de>
 <a239e97a-d1a1-388e-5db9-10758c33d702@redhat.com>
 <20180918142701.atfb4ul45k7tl6ew@linutronix.de>
 <7e9a13f3-93f5-fe4a-20d2-f4f9407bd43b@redhat.com>
 <83e271e1298d603c1105dd0dbea32d67da9cf1fa.camel@surriel.com>
 <36e8493f-f994-e885-8fe6-2f0d4a9904a1@redhat.com>
 <20180918160419.2zeru6xnufxixcax@linutronix.de>
 <11aa7d0f4ba36eff8b61a5dc1bd35ee5195fd576.camel@surriel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <c3492a53-74cd-4474-b664-f227ef37b7de@redhat.com>
Date:   Wed, 19 Sep 2018 07:55:51 +0200
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From: Paolo Bonzini <pbonzini@redhat.com>
To: Rik van Riel <riel@surriel.com>,
 Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
 Andy Lutomirski <luto@kernel.org>, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?=
 <rkrcmar@redhat.com>, kvm@vger.kernel.org,
 "Jason A. Donenfeld" <Jason@zx2c4.com>
Message-ID: <c3492a53-74cd-4474-b664-f227ef37b7de@redhat.com>
Subject: Re: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state
References: <20180912133353.20595-1-bigeasy@linutronix.de>
 <20180912133353.20595-5-bigeasy@linutronix.de>
 <8e5b64e4-b3e6-f884-beb6-b7b69ab2d8c1@redhat.com>
 <20180914203501.qibhpmueosvkr74w@linutronix.de>
 <a239e97a-d1a1-388e-5db9-10758c33d702@redhat.com>
 <20180918142701.atfb4ul45k7tl6ew@linutronix.de>
 <7e9a13f3-93f5-fe4a-20d2-f4f9407bd43b@redhat.com>
 <83e271e1298d603c1105dd0dbea32d67da9cf1fa.camel@surriel.com>
 <36e8493f-f994-e885-8fe6-2f0d4a9904a1@redhat.com>
 <20180918160419.2zeru6xnufxixcax@linutronix.de>
 <11aa7d0f4ba36eff8b61a5dc1bd35ee5195fd576.camel@surriel.com>
In-Reply-To: <11aa7d0f4ba36eff8b61a5dc1bd35ee5195fd576.camel@surriel.com>

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On 18/09/2018 19:29, Rik van Riel wrote:
> On Tue, 2018-09-18 at 18:04 +0200, Sebastian Andrzej Siewior wrote:
>> On 2018-09-18 17:29:52 [+0200], Paolo Bonzini wrote:
>>>> I don't think it matters what the PKRU state is
>>>> for kernel threads, since kernel PTEs should not
>>>> be using protection keys anyway.
>>>
>>> What about copy_from/to_user?
>>
>> This doesn't work for a kernel thread, does it? I mean they share the
>> init's MM and never do copy_{from|to}_user.
>=20
> Indeed, copy_from/to_user only works if current->mm
> points at an mm_struct with userspace memory.

A kthread can do use_mm/unuse_mm.

Paolo


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