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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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