Re: [Xen-devel] [V9 3/3] Differentiate IO/mem resources tracked by ioreq server

["Yu, Zhang" <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 1/6/2016 5:58 PM, Jan Beulich wrote:
> > > > On 06.01.16 at 10:44, <Paul.Durrant@xxxxxxxxxx> wrote:
> > >   -----Original Message-----
> > > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > > Sent: 06 January 2016 08:59
> > > To: Zhang Yu
> > > Cc: Andrew Cooper; Paul Durrant; Wei Liu; Ian Jackson; Stefano Stabellini;
> > > Kevin Tian; zhiyuan.lv@xxxxxxxxx; Shuai Ruan; xen-devel@xxxxxxxxxxxxx; Keir
> > > (Xen.org)
> > > Subject: Re: [Xen-devel] [V9 3/3] Differentiate IO/mem resources tracked by
> > > ioreq server
> > >
> > > > > > On 31.12.15 at 10:33, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > > > On 12/21/2015 10:45 PM, Jan Beulich wrote:
> > > > > > > > On 15.12.15 at 03:05, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> > > > > > @@ -2593,6 +2597,16 @@ struct hvm_ioreq_server
> > > *hvm_select_ioreq_server(struct domain *d,
> > > > > >            type = (p->type == IOREQ_TYPE_PIO) ?
> > > > > >                    HVMOP_IO_RANGE_PORT : HVMOP_IO_RANGE_MEMORY;
> > > > > >            addr = p->addr;
> > > > > > +        if ( type == HVMOP_IO_RANGE_MEMORY )
> > > > > > +        {
> > > > > > +             ram_page = get_page_from_gfn(d, p->addr >> PAGE_SHIFT,
> > > > > > +                                          &p2mt, P2M_UNSHARE);
> > > > > > +             if ( p2mt == p2m_mmio_write_dm )
> > > > > > +                 type = HVMOP_IO_RANGE_WP_MEM;
> > > > > > +
> > > > > > +             if ( ram_page )
> > > > > > +                 put_page(ram_page);
> > > > > > +        }
> > > > > You evaluate the page's current type here - what if it subsequently
> > > > > changes? I don't think it is appropriate to leave the hypervisor at
> > > > > the mercy of the device model here.
> > > > Well. I do not quite understand your concern. :)
> > > > Here, the get_page_from_gfn() is used to determine if the addr is a MMIO
> > > > or a write-protected ram. If this p2m type is changed, it should be
> > > > triggered by the guest and device model, e.g. this RAM is not supposed
> > > > to be used as the graphic translation table. And it should be fine.
> > > > But I also wonder, if there's any other routine more appropriate to get
> > > > a p2m type from the gfn?
> > > No, the question isn't the choice of method to retrieve the
> > > current type, but the lack of measures against the retrieved
> > > type becoming stale by the time you actually use it.
> > I don't think that issue is specific to this code. AFAIK nothing in the I/O
> > emulation system protects against a type change whilst a request is in
> > flight.
> > Also, what are the consequences of a change? Only that the wrong range type
> > is selected and the emulation goes to the wrong place. This may be a problem
> > for the VM but should cause no other problems.
> Okay, I buy this argument, but I think it would help if that was spelled
> out this way in the commit message.
Thank you, Paul & Jan. :)
A note will be added to explain this in the commit message in next
version.

Yu
> Jan
>
>
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