From: Rik van Riel <riel@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
X86 ML <x86@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
Paolo Bonzini <pbonzini@redhat.com>,
Ingo Molnar <mingo@redhat.com>,
Andrew Lutomirski <luto@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@suse.de>
Subject: Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until switch to userspace
Date: Mon, 03 Oct 2016 22:11:17 -0400 [thread overview]
Message-ID: <1475547077.21644.45.camel@redhat.com> (raw)
In-Reply-To: <CALCETrX_A71XYJxSxprd_ujHFUK_M7UfmCXeNnP1iGx17CSu8A@mail.gmail.com>
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On Mon, 2016-10-03 at 14:36 -0700, Andy Lutomirski wrote:
>
> Anything else that tries to read task xstate from memory, i.e. MPX
> and
> PKRU. (Although if we switch to eager-switched PKRU, then PKRU stops
> mattering for this purpose.)
>
> Actually, I don't see any way your patches can be compatible with
> PKRU
> without switching to eager-switched PKRU.
There is one case where the in-register PKRU state matters:
- user space accesses to memory
There are several cases where the in-memory PKRU state would
suffice:
- get_user_pages(_fast) to the local task (could also use registers)
- setting VMA/PTE permission bits (could also use registers)
There is one case where only in-memory PKRU state works, where
PKRU is currently simply ignored:
- get_user_pages to another task's memory
Dave, are there major obstacles to making read_pkru and write_pkru
work with in-memory state?
Would it be better for read/write_pkru to force the FPU state
to get loaded into registers, under the assumption that if things
like get_user_pages_fast happens, we will likely switch back to
userspace soon, anyway?
Would that assumption be wrong with KVM? :)
--
All Rights Reversed.
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next prev parent reply other threads:[~2016-10-04 2:11 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-01 20:31 [PATCH RFC 0/5] x86,fpu: make FPU context switching much lazier riel
2016-10-01 20:31 ` [PATCH RFC 1/5] x86,fpu: split prev/next task fpu state handling riel
2016-10-01 23:26 ` Andy Lutomirski
2016-10-02 0:02 ` Rik van Riel
2016-10-01 20:31 ` [PATCH RFC 2/5] x86,fpu: delay FPU register loading until switch to userspace riel
2016-10-01 23:44 ` Andy Lutomirski
2016-10-02 0:08 ` Rik van Riel
2016-10-03 20:54 ` Andy Lutomirski
2016-10-03 21:21 ` Rik van Riel
2016-10-03 21:36 ` Andy Lutomirski
2016-10-04 1:29 ` Rik van Riel
2016-10-04 2:09 ` Andy Lutomirski
2016-10-04 2:47 ` Rik van Riel
2016-10-04 3:02 ` Andy Lutomirski
2016-10-04 6:35 ` Ingo Molnar
2016-10-04 12:48 ` Rik van Riel
2016-10-04 2:11 ` Rik van Riel [this message]
2016-10-04 3:02 ` Andy Lutomirski
2016-10-02 0:42 ` Rik van Riel
2016-10-03 16:23 ` Dave Hansen
2016-10-01 20:31 ` [PATCH RFC 3/5] x86,fpu: add kernel fpu argument to __kernel_fpu_begin riel
2016-10-01 20:31 ` [PATCH RFC 4/5] x86,fpu: lazily skip FPU restore when still loaded riel
2016-10-03 20:04 ` Dave Hansen
2016-10-03 20:22 ` Rik van Riel
2016-10-03 20:49 ` Dave Hansen
2016-10-03 21:02 ` Rik van Riel
2016-10-01 20:31 ` [PATCH RFC 5/5] x86,fpu: kinda sorta fix up signal path riel
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