Re: [Xen-devel] Why gpa instead of mfn is directed used in emulated nic

[Tim Deegan <tim@xxxxxxx>]

At 10:14 +0800 on 06 Mar (1425633255), openlui wrote:
> At 2015-03-05 19:09:41, "Tim Deegan" <tim@xxxxxxx> wrote:
> >Hi,
> >
> >At 10:54 +0800 on 05 Mar (1425549262), openlui wrote:
> >> 2. From the trace info and qemu-dm's log, it seems that it is "GPA"
> >> (Guest Physical Address) instead of "MFN" in the IOREQ's data field
> >> received by qemu-dm:
> >
> >Yes.
> Thanks for your reply.
> 
> >> I think qemu-dm/rtl8139 should read/write data from "MFN" address in
> >> host memory instaed of GPA, and I find that there isn't hypercall
> >> from dom0 to "translate" the gpa to mfn in subsequent xentrace
> >> info. Is my understanding is wrong?  I would really appreciate your
> >> help.
> >
> >The hypercall that qemu uses to map the guest's memory for reading and
> >writing also takes a GFN/GPA.  So Qemu doesn't need to know what the
> >actual MFN is -- it just does all its operations in GPAs and Xen takes
> >care of the translations.
> 
> Could you give some hint about the type of hypercall qemu used to map the 
> guest's memory? I know that there are some xc_map_foreign_xxx interfaces
> in libxc which can do the similar work. However, it seems that these 
> interfaces should be given MFN instead of GFN/GPA.

That's some unfortunate historical APIs/docs. :(  The argument is in
fact a GFN in all cases, but for PV guests, which were implemented
first, that's == MFN.

Basically anything in the tools that deals with HVM guest memory
should _always_ be working in GFN/GPA.

Cheers,

Tim.

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