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Date: Tue, 19 May 2015 23:59:42 +0200
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From: Richard Weinberger <richard.weinberger@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH 00/23] userfaultfd v4
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: kvm <kvm@vger.kernel.org>, qemu-devel@nongnu.org, Sanidhya Kashyap <sanidhya.gatech@gmail.com>, Dave Hansen <dave.hansen@intel.com>, Andrea Arcangeli <aarcange@redhat.com>, zhang.zhanghailiang@huawei.com, Pavel Emelyanov <xemul@parallels.com>, Hugh Dickins <hughd@google.com>, Mel Gorman <mgorman@suse.de>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Andres Lagar-Cavilla <andreslc@google.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "open list:ABI/API" <linux-api@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, Johannes Weiner <hannes@cmpxchg.org>, Paolo Bonzini <pbonzini@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>, Peter Feiner <pfeiner@google.com>

On Tue, May 19, 2015 at 11:38 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Thu, 14 May 2015 19:30:57 +0200 Andrea Arcangeli <aarcange@redhat.com> wrote:
>
>> This is the latest userfaultfd patchset against mm-v4.1-rc3
>> 2015-05-14-10:04.
>
> It would be useful to have some userfaultfd testcases in
> tools/testing/selftests/.  Partly as an aid to arch maintainers when
> enabling this.  And also as a standalone thing to give people a
> practical way of exercising this interface.
>
> What are your thoughts on enabling userfaultfd for other architectures,
> btw?  Are there good use cases, are people working on it, etc?

UML is using SIGSEGV for page faults.
i.e. the UML processes receives a SIGSEGV, learns the faulting address
from the mcontext
and resolves the fault by installing a new mapping.

If userfaultfd is faster that the SIGSEGV notification it could speed
up UML a bit.
For UML I'm only interested in the notification, not the resolving
part. The "missing"
data is present, only a new mapping is needed. No copy of data.

Andrea, what do you think?

-- 
Thanks,
//richard