Re: [KVM paravirt issue?] Re: vsyscall=emulate regression

[Avi Kivity <avi@redhat.com>]

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