Re: How to share a page between dom0 and Hypervisror

[veerasena reddy <veeruyours@gmail.com>]

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Hi Ian,

Thanks a lot for the detailed explanation.
You were correct, I had to use "my_rd_wr_page" not "&my_rd_wr_page". I also
had to change my code to translate the PFN to MFN (pfn_to_mfn()) on dom0
kernel itself before passing it to hypercall.
Now it works.

Thanks & Regards,
VSR.

On Mon, Jun 6, 2011 at 7:23 PM, Ian Campbell <Ian.Campbell@citrix.com>wrote:

> On Mon, 2011-06-06 at 14:28 +0100, veerasena reddy wrote:
> > Hi,
> >
> > In one of experiments, I need to map a page allocated in dom0 to
> > hypervisor and access/modify the page contents in hypervisor.
> > I tried this by adding a new hypercall, and pass the GPA of the page
> > to its handler in hypervisor which does the following:
> >
> > ==================== Hypercall handler  ================
> > DO(my_rd_wr)(int cmd, XEN_GUEST_HANDLE(void) arg)
> > {
> >         unsigned long dom0_gpa;
> >         unsigned long gmfn;
> >         unsigned long mfn;
> >         void *my_rd_wr_page;
> >         struct domain *d = current->domain;
> >
> >         printk(XENLOG_G_DEBUG "%s:L%u: Entered\n", __FUNCTION__,
> __LINE__);
> >         switch( cmd )
> >         {
> >                 case 0x1:
> >                         if ( copy_from_guest(&dom0_gpa, arg, 1) )
> >                                 return -EFAULT;
> >                         printk(XENLOG_G_DEBUG "%s:L%u: GPA read 0x%lx\n",
> >                                                 __FUNCTION__, __LINE__,
> dom0_gpa);
> >
> >                         gmfn = dom0_gpa >> 12;
> >                         mfn = gmfn_to_mfn(d, gmfn);
> >                         if ( !mfn_valid(mfn) ||
> >                              !get_page_and_type(mfn_to_page(mfn), d,
> PGT_writable_page) )
> >                         {
> >                                 printk(XENLOG_G_WARNING
> >                                         "Bad GMFN %lx (MFN %lx)\n", gmfn,
> mfn);
> >                                 return 0;
> >                         }
> >
> >                         my_rd_wr_page = map_domain_page(mfn);
> >
> >                         /* Do your initialization of the page here; just
> write '2' in all bytes */
> >                         memset(my_rd_wr_page, 2, 1<<12);
> >
> >                         unmap_domain_page(my_rd_wr_page);
> >                         put_page_and_type(mfn_to_page(mfn));
> >                         break;
> >
> >                 default:
> >                         printk(XENLOG_G_DEBUG "%s:L%u: unhandled\n",
> >                                                 __FUNCTION__, __LINE__);
> >                         break;
> >         }
> >
> >         return 0;
> > }
> > ============================
> >
> > I have allocated a page from a sample dom0 kernel module (using
> vmalloc()), and passed the physical address of it to hypercall.
> >
> > void my_rd_wr_page_setup(void)
> > {
> >     unsigned long my_gpa;
> >     int err;
> >     char *my_rd_wr_page = NULL;
> >
> >     my_rd_wr_page     = __vmalloc(
> >                         1 * PAGE_SIZE,
> >                         GFP_KERNEL | __GFP_HIGHMEM,
> >                         __pgprot(__PAGE_KERNEL & ~_PAGE_NX));
> >
> >     my_gpa = vmalloc_to_pfn((char *)&my_rd_wr_page) << PAGE_SHIFT;
>
> &my_rd_wr_page is the address of the variable (i.e. probably a pointer
> into the current stack) and not the address of the page you are trying
> to reference.
>
> Secondly vmalloc_to_pfn will return you a guest physical address, while
> hypercalls from PV guests always take an MFN.
>
> On the hypercall side your call to gmfn_to_mfn is normally an identity
> function for a PV guest which map_domain_page takes a machine address.
>
> So I think you need to launder the address through the p2m in the kernel
> before passing it to the hypercall.
>
> >     printk("%s: Before Hypercall; my_rd_wr_page=%p my_gpa=%lx\n",
> __FUNCTION__, my_rd_wr_page, my_gpa);
> >     memset(my_rd_wr_page, 0, PAGE_SIZE);
> >     err = _hypercall2(int, my_rd_wr, 0x1, (void *)&my_gpa);
> >     printk("%s: Hypercall returned; errno-%d\n", __FUNCTION__, err);
> > }
> >
> >
> > When I loaded the module, the following error has been observed:
> >
> > =============== On dom0 ===========
> > xen_features[0].writable_page_tables = 0
> > xen_features[0].writable_descriptor_tables = 0
> > xen_features[0].auto_translated_physmap = 0
> > xen_features[0].supervisor_mode_kernel = 0
> > xen_features[0].pae_pgdir_above_4gb = 1
> > my_rd_wr_page_setup: Before Hypercall; my_rd_wr_page=ffffc90010f7e000
> my_gpa=d7cf000
> > my_rd_wr_page_setup: Hypercall returned; errno-0
> > =====================================
> > On hypervisor:
> > (XEN) do_my_rd_wr:L178: GPA read 0xd7cf000
> > (XEN) mm.c:2037:d0 Error pfn d7cf: rd=ffff8300773c0000,
> od=0000000000000000, caf=180000000000000, taf=0000000000000000
> > (XEN) Bad GMFN d7cf (MFN d7cf)
> > ====================================
> >
> > From the dom0 messages (where i read the features), it looks like
> writable_page_tables is not set. Is it causing the issue in my case.
>
> Writable page tables is something else, it relates to how a _guest_ can
> update it's own page tables, not how Xen creates mappings of things.
>
> >
> > Could you please advice if i did something wrong here.
> > You are most welcome if you have completely different approach which
> works;
> >
> > Thanks a lot in advance
> >
> > Regards,
> > VSR.
>
>
>

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