[Xen-devel] How to share a page between dom0 and Hypervisror

[veerasena reddy <veeruyours@xxxxxxxxx>]

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;
    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.

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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