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    SubjectRe: [KVM paravirt issue?] Re: vsyscall=emulate regression
    On 02/15/2012 09:36 PM, Andy Lutomirski wrote:
    > Hi, kvm people-
    >
    > Here's a strange failure. It could be a bug in something
    > RHEL6-specific, but it could be a generic issue that only triggers
    > with a paravirt guest with old userspace on a non-ept host. There was
    > a bug like this on Xen, and I'm wondering something's wrong on kvm as
    > well.
    >
    > For background, a change in 3.1 (IIRC) means that, when
    > vsyscall=emulate or vsyscall=none, the vsyscall page in the fixmap is
    > NX. It seems like Amit's machine is marking the physical PTE present
    > but unreadable.

    No such thing as present and unreadable, without EPT.

    > So I could have messed up, or there could be a subtle
    > bug somewhere. Any ideas?

    What's the code trying to do? Execute an instruction from an
    non-executable page, trap the #PF, and emulate? And what are the
    symptoms? wrong error code for the #PF? That could easily be a kvm bug.

    >
    > I'll try to reproduce on a non-ept host later on, but that will
    > involve finding one.

    rmmod kvm-intel
    moprobe kvm-intel ept=0

    > Hmm. You don't have ept. If your guest kernel supports paravirt,
    > then you might use the hypercall interface instead of programming the
    > fixmap directly.

    There is no hypercall interface for writing page tables in kvm.

    >
    > >
    > > This is what I get with vsyscall=none, where emulate and native work
    > > fine on the 3.2 kernel on different host hardware, the guest stays the
    > > same:
    > >
    > >
    > > [ 2.874661] debug: unmapping init memory ffffffff8167f000..ffffffff818dc000
    > > [ 2.876778] Write protecting the kernel read-only data: 6144k
    > > [ 2.879111] debug: unmapping init memory ffff880001318000..ffff880001400000
    > > [ 2.881242] debug: unmapping init memory ffff8800015a0000..ffff880001600000
    > > [ 2.884637] init[1] vsyscall attempted with vsyscall=none ip:ffffffffff600400 cs:33 sp:7fff2f48fe18 ax:7fff2f48fe50 si:7fff2f48ff08 di:0
    >
    > This like (vsyscall attempted) means that the emulation worked
    > correctly. Your other traces didn't have it or anything like it,
    > which mostly rules out do_emulate_vsyscall issues.
    >

    Can you point me at the code in question?

    Amit, a trace would be nice.

    --
    error compiling committee.c: too many arguments to function



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