linux-kernel - Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until switch to userspace

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Message-ID: <1475547077.21644.45.camel@redhat.com>
Date:   Mon, 03 Oct 2016 22:11:17 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until
 switch to userspace

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? :)

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