Re: [Xen-devel] Migration bug added by commit 2df1aa01bef7366798248ac6d03cfb42048b003d

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Paul Durrant
> Sent: 29 June 2015 10:42
> To: Don Slutz; xen-devel@xxxxxxxxxxxxx; Jan Beulich
> Subject: Re: [Xen-devel] Migration bug added by commit
> 2df1aa01bef7366798248ac6d03cfb42048b003d
> 
> > -----Original Message-----
> > From: Don Slutz [mailto:don.slutz@xxxxxxxxx]
> > Sent: 27 June 2015 22:02
> > To: xen-devel@xxxxxxxxxxxxx; Paul Durrant; Jan Beulich
> > Subject: Migration bug added by commit
> > 2df1aa01bef7366798248ac6d03cfb42048b003d
> >
> > commit 2df1aa01bef7366798248ac6d03cfb42048b003d
> > Author: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Date:   Tue Jun 23 18:07:49 2015 +0200
> >
> >      x86/hvm: remove hvm_io_pending() check in hvmemul_do_io()
> >
> > ...
> > -            rc = X86EMUL_RETRY;
> > -            if ( !hvm_send_assist_req(s, &p) )
> > +            rc = hvm_send_assist_req(s, &p);
> > +            if ( rc != X86EMUL_RETRY )
> > ...
> >       if ( unlikely(!vcpu_start_shutdown_deferral(curr)) )
> > -        return 0; /* implicitly bins the i/o operation */
> > +        return X86EMUL_OKAY;
> >
> > So now X86EMUL_OKAY is returned from hvmemul_do_io() during
> > shutdown.
> >
> >  From Jan Beulich about this:
> >
> >
> >   Re: [Xen-devel] [PATCH 3/5] hvmemul_do_io: If the send to the ioreq
> >   server failed do not retry.
> >   <https://www.mail-archive.com/search?l=xen-
> > devel@xxxxxxxxxxxxx&q=subject:%22Re%5C%3A+%5C%5BXen%5C-
> >
> devel%5C%5D+%5C%5BPATCH+3%5C%2F5%5C%5D+hvmemul_do_io%5C%3
> >
> A+If+the+send+to+the+ioreq+server+failed+do+not+retry.%22&o=newest
> > >
> >
> >
> > Jan Beulich
> > <https://www.mail-archive.com/search?l=xen-
> > devel@xxxxxxxxxxxxx&q=from:%22Jan+Beulich%22>
> > Fri, 30 Jan 2015 02:24:55 -0800
> > <https://www.mail-archive.com/search?l=xen-
> > devel@xxxxxxxxxxxxx&q=date:20150130>
> >
> >
> > >>> On 30.01.15 at 01:52, <dsl...@xxxxxxxxxxx> wrote:
> > > I.E. do just what no backing DM does.
> >
> >
> > _If_ this is correct, the if() modified here should be folded with the
> > one a few lines up. But looking at the description of the commit that
> > introduced this (bac0999325 "x86 hvm: Do not incorrectly retire an
> > instruction emulation...", almost immediately modified by f20f3c8ece
> > "x86 hvm: On failed hvm_send_assist_req(), io emulation...") I doubt
> > this is really what we want, or at the very least your change
> > description should explain what was wrong with the original commit.
> >
> > Jan
> >
> >
> >
> > going up the call stack:
> >
> > in handle_pio when hvmemul_do_pio_buffer() returns X86EMUL_OKAY, it
> > returns 1.
> >
> >     svm_vmexit_handler 2578 if ( handle_pio(port, bytes, dir) )
> > or
> >     vmx_vmexit_handler 3178 if ( handle_pio(port, bytes, dir) )
> >
> > both update the IP in this case which is wrong during shutdown.
> >
> 
> If this is indeed the problem then it seems to me that the correct fix is to 
> add
> a check in handle_pio() and return 0 if the cpu is shutting down.
> 

Actually, with some more thought, it sounds like we essentially want to unwind 
the I/O emulation if we're shutting down and I think this could be achieved by 
having hvm_send_assist_req() return X86EMUL_RETRY if it hits the shutdown 
deferral case, having hvm_emul_do_io() reset the emulation state back to 'none' 
if the domain is shutting down, and then having handle_pio() return 0 if sees 
X86EMUL_RETRY without any form of completion being requested.

  Paul

>   Paul
> 
> > So I think:
> >
> >              rc = hvm_send_assist_req(s, &p);
> >              if ( rc != X86EMUL_RETRY )
> >                   vio->io_state = HVMIO_none;
> >
> > needs to change to:
> >
> >                  rc = hvm_send_assist_req(s, &p);
> >                  if ( rc != X86EMUL_RETRY )
> >                  {
> >                      vio->io_state = HVMIO_none;
> >                      if ( rc == X86EMUL_OKAY )
> >                          rc = X86EMUL_RETRY;
> >                  }
> >
> >
> >     -Don Slutz
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