Re: [Xen-devel] [PATCH v5 01/16] x86/hvm: make sure emulation is retried if domain is shutting down

[Don Slutz <don.slutz@xxxxxxxxx>]

On 06/30/15 09:45, Andrew Cooper wrote:
> On 30/06/15 14:05, Paul Durrant wrote:
> > The addition of commit 2df1aa01 "x86/hvm: remove hvm_io_pending() check
> > in hvmemul_do_io()" causes a problem in migration because I/O that was
> > caught by the test of vcpu_start_shutdown_deferral() in
> > hvm_send_assist_req() is now considered completed rather than requiring
> > a retry.
> >
> > This patch fixes the problem by having hvm_send_assist_req() return
> > X86EMUL_RETRY rather than X86EMUL_OKAY if the
> > vcpu_start_shutdown_deferral() test fails and then making sure that
> > the emulation state is reset if the domain is found to be shutting
> > down.
> >
> > Reported-by: Don Slutz <don.slutz@xxxxxxxxx>
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Keir Fraser <keir@xxxxxxx>
> > Jan Beulich <jbeulich@xxxxxxxx>
> > Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> > ---
> >   xen/arch/x86/hvm/emulate.c |    2 +-
> >   xen/arch/x86/hvm/hvm.c     |    2 +-
> >   2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
> > index fe5661d..8b60843 100644
> > --- a/xen/arch/x86/hvm/emulate.c
> > +++ b/xen/arch/x86/hvm/emulate.c
> > @@ -183,7 +183,7 @@ static int hvmemul_do_io(
> >           else
> >           {
> >               rc = hvm_send_assist_req(s, &p);
> > -            if ( rc != X86EMUL_RETRY )
> > +            if ( rc != X86EMUL_RETRY || curr->domain->is_shutting_down )
Having spent more time thinking about this, a safer test is "!curr-> 
defer_shutdown" .
The reason are as follows:
1) This uses what vcpu_start_shutdown_deferral() returns.  But it is 
only valid if
     vcpu_start_shutdown_deferral() is called which is the case now.
2) The checking of "curr->domain->is_shutting_down" without also
     checking for "curr-> defer_shutdown" may open a timing window.
3) No use of "spin_lock(&d->shutdown_lock)".


> >                   vio->io_state = HVMIO_none;
> >               else if ( data_is_addr || dir == IOREQ_WRITE )
> >                   rc = X86EMUL_OKAY;
> > diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> > index 535d622..f0cf064 100644
> > --- a/xen/arch/x86/hvm/hvm.c
> > +++ b/xen/arch/x86/hvm/hvm.c
> > @@ -2613,7 +2613,7 @@ int hvm_send_assist_req(struct hvm_ioreq_server *s, 
> > ioreq_t *proto_p)
> >   
> >       ASSERT(s);
> >       if ( unlikely(!vcpu_start_shutdown_deferral(curr)) )
> > -        return X86EMUL_OKAY;
> > +        return X86EMUL_RETRY;
I will also note that this change makes hvm_send_assist_req() have only 
2 return codes:
X86EMUL_RETRY
X86EMUL_UNHANDLEABLE

and so is not clear why its return should not be a bool.

And there are now 2 reasons why X86EMUL_RETRY is returned, which require 
the caller to redo a test.
Granted X86EMUL_OKAY is bad.  But I wounder if it would be better to add 
an enum for
hvm_send_assist_req() to return in stead of overloading the usage of 
X86EMUL_UNHANDLEABLE
and X86EMUL_RETRY with yet more meanings.

   -Don Slutz

> >   
> >       list_for_each_entry ( sv,
> >                             &s->ioreq_vcpu_list,
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