Re: [Xen-devel] [RFC Patch v4 2/8] ioreq: bump the number of IOREQ page to 4 pages

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf
> Of Paul Durrant
> Sent: 06 December 2017 16:10
> To: 'Chao Gao' <chao.gao@xxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Tim
> (Xen.org) <tim@xxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>;
> xen-devel@xxxxxxxxxxxxx; Jan Beulich <jbeulich@xxxxxxxx>; Ian Jackson
> <Ian.Jackson@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [RFC Patch v4 2/8] ioreq: bump the number of
> IOREQ page to 4 pages
> 
> > -----Original Message-----
> > From: Chao Gao [mailto:chao.gao@xxxxxxxxx]
> > Sent: 06 December 2017 09:02
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxx; Tim (Xen.org) <tim@xxxxxxx>; Stefano
> > Stabellini <sstabellini@xxxxxxxxxx>; Konrad Rzeszutek Wilk
> > <konrad.wilk@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; George
> > Dunlap <George.Dunlap@xxxxxxxxxx>; Andrew Cooper
> > <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; Ian Jackson
> > <Ian.Jackson@xxxxxxxxxx>
> > Subject: Re: [RFC Patch v4 2/8] ioreq: bump the number of IOREQ page to 4
> > pages
> >
> > On Wed, Dec 06, 2017 at 03:04:11PM +0000, Paul Durrant wrote:
> > >> -----Original Message-----
> > >> From: Chao Gao [mailto:chao.gao@xxxxxxxxx]
> > >> Sent: 06 December 2017 07:50
> > >> To: xen-devel@xxxxxxxxxxxxx
> > >> Cc: Chao Gao <chao.gao@xxxxxxxxx>; Paul Durrant
> > >> <Paul.Durrant@xxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>; Stefano
> > Stabellini
> > >> <sstabellini@xxxxxxxxxx>; Konrad Rzeszutek Wilk
> > >> <konrad.wilk@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; George
> > >> Dunlap <George.Dunlap@xxxxxxxxxx>; Andrew Cooper
> > >> <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; Ian
> > Jackson
> > >> <Ian.Jackson@xxxxxxxxxx>
> > >> Subject: [RFC Patch v4 2/8] ioreq: bump the number of IOREQ page to 4
> > >> pages
> > >>
> > >> One 4K-byte page at most contains 128 'ioreq_t'. In order to remove the
> > vcpu
> > >> number constraint imposed by one IOREQ page, bump the number of
> > IOREQ
> > >> page to
> > >> 4 pages. With this patch, multiple pages can be used as IOREQ page.
> > >>
> > >> Basically, this patch extends 'ioreq' field in struct hvm_ioreq_server to
> an
> > >> array. All accesses to 'ioreq' field such as 's->ioreq' are replaced with
> > >> FOR_EACH_IOREQ_PAGE macro.
> > >>
> > >> In order to access an IOREQ page, QEMU should get the gmfn and map
> > this
> > >> gmfn
> > >> to its virtual address space.
> > >
> > >No. There's no need to extend the 'legacy' mechanism of using magic
> page
> > gfns. You should only handle the case where the mfns are allocated on
> > demand (see the call to hvm_ioreq_server_alloc_pages() in
> > hvm_get_ioreq_server_frame()). The number of guest vcpus is known at
> > this point so the correct number of pages can be allocated. If the creator 
> > of
> > the ioreq server attempts to use the legacy hvm_get_ioreq_server_info()
> > and the guest has >128 vcpus then the call should fail.
> >
> > Great suggestion. I will introduce a new dmop, a variant of
> > hvm_get_ioreq_server_frame() for creator to get an array of gfns and the
> > size of array. And the legacy interface will report an error if more
> > than one IOREQ PAGES are needed.
> 
> You don't need a new dmop for mapping I think. The mem op to map ioreq
> server frames should work. All you should need to do is update
> hvm_get_ioreq_server_frame() to deal with an index > 1, and provide some
> means for the ioreq server creator to convert the number of guest vcpus into
> the correct number of pages to map. (That might need a new dm op).

I realise after saying this that an emulator already knows the size of the 
ioreq structure and so can easily calculate the correct number of pages to map, 
given the number of guest vcpus.

  Paul

> 
>   Paul
> 
> >
> > Thanks
> > Chao
> 
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