Working out how to write to the virtual console from a guest domain was
harder for me than I expected. Once you have console output, figuring
out the rest is easy, right?
Here is the result. It is a one file xen guest kernel, in the spirit
of mini-os, that writes a message to the Xen-2.0 virtual console.
Feel free to add it to the extras directory if it looks useful.
/*
* hello.c - xen user domain kernel that demonstrates console output
*
* compile with
cc -nostdlib -Wl,-Ttext,0xc0100000 -D__KERNEL__
-Ixen-2.0.bk/linux-2.6.10-xenU/include
-Ixen-2.0.bk/linux-2.6.10-xenU/include/asm-xen -o hellokernel hello.c
* run demo with:
dom0$ xm create -c kernel=hellokernel
Using config file "/etc/xen/xmdefconfig".
Started domain xmdefconfig, console on port 9652
************ REMOTE CONSOLE: CTRL-] TO QUIT ********
hello, world
farewell cruel world
************ REMOTE CONSOLE EXITED *****************
*/
#include <asm/pgtable.h>
#include <asm-xen/ctrl_if.h>
#include <asm-xen/evtchn.h>
#include <asm-xen/hypervisor.h>
start_info_t *HYPERVISOR_start_info;
extern char shared_info[PAGE_SIZE];
/* _start is the default name ld will use as the entry point. When xen
* loads the domain, it will start execution at the elf entry point.
*/
void _start()
{
/*
* Grab start_info
*/
/* The linux build setup_guest() put a start_info_t* into %esi.
* =S is inline asm code for get output from reg %esi.
*/
asm("":"=S" (HYPERVISOR_start_info));
/*
* Try real console
*/
/* If the xen hypervisor was compiled with ''make verbose=y''
* or ''make debug=y'', the emergency console IO call
* will write to the real console (serial line or VGA)
* from a guest domain. Otherwise when called from domU, the
* console_io calls will be ignored. dom0 can always write
* to the real console.
*/
char realhello[] = "hello, world on the real console\n";
HYPERVISOR_console_io(CONSOLEIO_write, sizeof(realhello), realhello);
/*
* Try virtual console
*/
/* To write to the xen virtual console, we need to map in the
* shared page used by the the domain controller interface. The
* HYPERVISOR_start_info struct identifies the page table and
* shared_info pages.
*
* The following code maps the shared_info mfn (machine frame number)
* into this domains address space over the shared_info[] page.
*/
/*
* map shared_info page
*/
/* The pgd page (page global directory - level 2 page table) is
* constructed by setup_guest() in tools/libxc/xc_linux_build.c
* Lookup the machine address of ptetab in pgd to construct the
* machine address of the pte entry for shared_info,
* and then call mmu_update to change mapping.
*/
pgd_t *pgd = (pgd_t*)HYPERVISOR_start_info->pt_base;
int ptetab_ma = pgd_val(pgd[pgd_index((unsigned long)shared_info)])
& (PAGE_MASK);
int idx = pte_index((unsigned long)shared_info);
int pte_ma = ptetab_ma + (idx*sizeof(pte_t));
mmu_update_t req;
req.ptr = pte_ma;
req.val = HYPERVISOR_start_info->shared_info|7;
HYPERVISOR_mmu_update(&req, 1, NULL);
/*
* Setup control interface
*/
control_if_t *ctrl_if = ((control_if_t *)((char *)shared_info + 2048));
int ctrl_if_evtchn = HYPERVISOR_start_info->domain_controller_evtchn;
/*
* Put message on the control interface ring and trigger virtual
* console writer.
*/
ctrl_msg_t *msg = ctrl_if->tx_ring;
char hello[] = "hello, world\n\r";
msg->type = CMSG_CONSOLE;
msg->subtype = CMSG_CONSOLE_DATA;
msg->length = sizeof hello;
memcpy(msg->msg, hello, sizeof(hello)+1);
++ctrl_if->tx_req_prod;
notify_via_evtchn(ctrl_if_evtchn);
char msg2[] = "farewell cruel world\n\r";
++msg;
msg->type = CMSG_CONSOLE;
msg->subtype = CMSG_CONSOLE_DATA;
msg->length = sizeof msg2;
memcpy(msg->msg, msg2, sizeof(msg2)+1);
++ctrl_if->tx_req_prod;
notify_via_evtchn(ctrl_if_evtchn);
HYPERVISOR_shutdown();
}
/* Create shared_info page. This page is mapped over by the real shared
* info page
*/
asm(".align 0x1000; shared_info:;.skip 0x1000;");
/* emit the elf segment Xen builder expects in kernel image */
asm(".section __xen_guest;"
".ascii
\"GUEST_OS=linux,GUEST_VER=2.6,XEN_VER=2.0,VIRT_BASE=0xC0000000\";"
".ascii \",LOADER=generic\";"
".ascii \",PT_MODE_WRITABLE\";"
".byte 0;"
);
-------------------------------------------------------
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