Re: [Xen-devel] [PATCH v2 2/3] x86/ioreq server: Rename p2m_mmio_write_dm to p2m_ioreq_server

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

On 4/21/2016 9:31 PM, Paul Durrant wrote:
> > -----Original Message-----
> > From: Yu, Zhang [mailto:yu.c.zhang@xxxxxxxxxxxxxxx]
> > Sent: 21 April 2016 13:25
> > To: George Dunlap; Paul Durrant; Jan Beulich; Wei Liu
> > Cc: Kevin Tian; Keir (Xen.org); Andrew Cooper; Tim (Xen.org); xen-
> > devel@xxxxxxxxxxxxx; zhiyuan.lv@xxxxxxxxx; jun.nakajima@xxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH v2 2/3] x86/ioreq server: Rename
> > p2m_mmio_write_dm to p2m_ioreq_server
> >
> >
> >
> > On 4/21/2016 1:06 AM, George Dunlap wrote:
> > > On 20/04/16 17:58, Paul Durrant wrote:
> > > > > -----Original Message-----
> > > > > From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf
> > Of Jan
> > > > > Beulich
> > > > > Sent: 20 April 2016 17:53
> > > > > To: George Dunlap; Paul Durrant; Wei Liu; yu.c.zhang@xxxxxxxxxxxxxxx
> > > > > Cc: Kevin Tian; Keir (Xen.org); Andrew Cooper; Tim (Xen.org); xen-
> > > > > devel@xxxxxxxxxxxxx; zhiyuan.lv@xxxxxxxxx; jun.nakajima@xxxxxxxxx
> > > > > Subject: Re: [Xen-devel] [PATCH v2 2/3] x86/ioreq server: Rename
> > > > > p2m_mmio_write_dm to p2m_ioreq_server
> > > > >
> > > > > > > > George Dunlap <george.dunlap@xxxxxxxxxx> 04/20/16 6:30 PM >>>
> > > > > > On Wed, Apr 20, 2016 at 4:02 PM, George Dunlap
> > > > > <george.dunlap@xxxxxxxxxx> wrote:
> > > > > > > On 19/04/16 12:02, Yu, Zhang wrote:
> > > > > > > > So I suppose the only place we need change for this patch is
> > > > > > > > for hvmmem_type_t, which should be defined like this?
> > > > > > > >
> > > > > > > > typedef enum {
> > > > > > > >     HVMMEM_ram_rw,             /* Normal read/write guest RAM */
> > > > > > > >     HVMMEM_ram_ro,             /* Read-only; writes are discarded */
> > > > > > > >     HVMMEM_mmio_dm,            /* Reads and write go to the device
> > > > > model */
> > > > > > > > #if __XEN_INTERFACE_VERSION__ >= 0x00040700
> > > > > > > >     HVMMEM_ioreq_server
> > > > > > > > #else
> > > > > > > >     HVMMEM_mmio_write_dm
> > > > > > > > #endif
> > > > > > > > } hvmmem_type_t;
> > > > > > > >
> > > > > > > > Besides, does 4.7 still accept freeze exception? It would be great
> > > > > > > > if we can get an approval for this.
> > > > > > > Wait, do we *actually* need this?  Is anyone actually using this?
> > > > > > >
> > > > > > > I'd say remove it, and if anyone complains, *then* do the #ifdef'ery
> > as
> > > > > > > a bug-fix.  I'm pretty sure that's Linux's policy -- You Must Keep
> > > > > > > Userspace Working, but you can break it to see if anyone complains
> > first.
> > > > > We don't normally do it like that - we aim at keeping things compatible
> > > > > right away. I don't know of a case where we would have knowingly
> > broken
> > > > > compatibility for users of the public headers (leaving aside tool stack only
> > > > > stuff of course).
> > > > >
> > > > > > Going further than this:
> > > > > >
> > > > > > The proposed patch series not only changes the name, it changes the
> > > > > > functionality.  We do not want code to *compile* against 4.7 and then
> > > > > > not *work* against 4.7; and the worst of all is to compile and sort of
> > > > > > work but do it incorrectly.
> > > > > I had the impression that the renaming patch was what it is - a renaming
> > > > > patch, without altering behavior.
> > > > >
> > > > > > Does the ioreq server have a way of asking Xen what version of the ABI
> > > > > > it's providing?  I'm assuming the answer is "no"; in which case code
> > > > > > that is compiled against the 4.6 interface but run on a 4.8 interface
> > > > > > that looks like this will fail in a somewhat unpredictable way.
> > > > > The only thing it can do is ask for the Xen version. The ABI version is not
> > > > > being returned by anything (but perhaps should be).
> > > > >
> > > > > > Given that:
> > > > > >
> > > > > > 1. When we do check the ioreq server functionality in, what's the
> > > > > > correct way to deal with code that wants to use the old interface, and
> > > > > > what do we do with code compiled against the old interface but
> > running
> > > > > > on the new one?
> > > > > For the full series I'm not sure I can really tell.But as said, for the rename
> > > > > patch alone I thought it is just a rename. And that's what we want to get
> > > > > in (see Paul's earlier reply - he wants to see the old name gone, so it
> > won't
> > > > > be used any further).
> > > > >
> > > > > > 2. What's the best thing to do for this release?
> > > > > If the entire series (no matter whether to go in now or later) is changing
> > > > > behavior, then the only choice is to consider the currently used enum
> > > > > value burnt, and use a fresh one for the new semantics.
> > > > It sounds like that would be best way. If we don't so that then we have to
> > maintain the write-dm semantics for pages of that type unless the type is
> > claimed (by using the new hypercall) and that's bit icky. I much prefer that
> > pages of the new type are treated as RAM until claimed.
> > > I think the only sensible way to keep the enum is to also keep the
> > > functionality, which would mean using *another* p2m type for
> > ioreq_server.
> > > Given that the functionality isn't going away for 4.7, I don't see an
> > > urgent need to remove the enum; but if Paul does, then a patch renaming
> > > it to HVMMEM_unused would be the way forward then I guess.  Once the
> > > underlying p2m type goes away, you'll want to return -EINVAL for this
> > > enum value.
> > So the enum would be sth. like this?
> >
> > typedef enum {
> >      HVMMEM_ram_rw,        /* Normal read/write guest RAM */
> >      HVMMEM_ram_ro,        /* Read-only; writes are discarded */
> >      HVMMEM_mmio_dm,       /* Reads and write go to the device model */
> > #if __XEN_INTERFACE_VERSION__ < 0x00040700
> >      HVMMEM_mmio_write_dm, /* Read-only; writes go to the device model
> > */
> > #else
> >      HVMMEM_unused,
> > #endif
> >      HVMMEM_ioreq_server
> > } hvmmem_type_t;
> I believe that's correct, but presumably there's need to be a change to the 
> hypervisor since any reference there to HVMMEM_mmio_write_dm (which I think is 
> limited to the get and set mem type code in hvm.c) will now need to map 
> HVMMEM_unused to the old p2m_mmio_write_dm type.
Thank you, Paul.

But p2m_mmio_write_dm will not exist any more...
E.g. if in hvmop_get_mem_type(), if type 0xf(p2m_ioreq_server) is
returned, we could just return HVMMEM_ioreq_server. No need to
worry about the HVMMEM_mmio_write_dm.

Maybe we only need to change the beginning of hvmop_set_mem_type()
to sth. like this:

/* Interface types to internal p2m types */
static const p2m_type_t memtype[] = {
    [HVMMEM_ram_rw]  = p2m_ram_rw,
    [HVMMEM_ram_ro]  = p2m_ram_ro,
    [HVMMEM_mmio_dm] = p2m_mmio_dm,
    [HVMMEM_unused] = p2m_invalid,  /* this will be rejected later */
    [HVMMEM_ioreq_server] = p2m_ioreq_server
};
and later in the same routine, just reject the HVMMEM_unused type, in
an if(with unlikely) statement.

>   Paul
B.R.
Yu

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