Re: [Qemu-devel] [PATCH v2 1/2] Generalize -machine command line option

[Anthony PERARD <anthony.perard@citrix.com>]

On Wed, 25 May 2011, Ian Campbell wrote:

> On Tue, 2011-05-24 at 18:18 +0200, Jan Kiszka wrote:
> > On 2011-05-24 18:06, Ian Campbell wrote:
> > > On Sun, 2011-05-22 at 13:00 +0200, Jan Kiszka wrote:
> > >> From: Jan Kiszka <jan.kiszka@siemens.com>
> > >>
> > >> -machine somehow suggests that it selects the machine, but it doesn't.
> > >> Fix that before this command is set in stone.
> > >>
> > >> Actually, -machine should supersede -M and allow to introduce arbitrary
> > >> per-machine options to the command line. That will change the internal
> > >> realization again, but we will be able to keep the user interface
> > >> stable.
> > >>
> > >> CC: Anthony PERARD <anthony.perard@citrix.com>
> > >> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> > >
> > > "-machine xenfv" doesn't work for me with this patch, it gives:
> > >         Program received signal SIGSEGV, Segmentation fault.
> > >         hypercall_buffer_cache_lock (xch=0x0) at xc_hcall_buf.c:36
> > >         36	xc_hcall_buf.c: No such file or directory.
> > >         	in xc_hcall_buf.c
> > >         (gdb) bt
> > >         #0  hypercall_buffer_cache_lock (xch=0x0) at xc_hcall_buf.c:36
> > >         #1  0xb7d53f1d in hypercall_buffer_cache_alloc (xch=0x0, b=0xbffff77c, nr_pages=1) at xc_hcall_buf.c:52
> > >         #2  xc__hypercall_buffer_alloc_pages (xch=0x0, b=0xbffff77c, nr_pages=1) at xc_hcall_buf.c:128
> > >         #3  0xb7d54028 in xc__hypercall_buffer_alloc (xch=0x0, b=0xbffff77c, size=16) at xc_hcall_buf.c:162
> > >         #4  0xb7d42719 in xc_get_hvm_param (handle=0x0, dom=248, param=5, value=0xbffff810) at xc_domain.c:1078
> > >         #5  0x08252777 in xen_hvm_init () at /home/ianc/devel/qemu.git/xen-all.c:803
> > >         #6  0x082921e9 in pc_xen_hvm_init (ram_size=536870912, boot_device=0xbffffabb "cda", kernel_filename=0x0, kernel_cmdline=0x82d648f "", initrd_filename=0x0, cpu_model=0x0) at /home/ianc/devel/qemu.git/hw/pc_piix.c:246
> > >         #7  0x081e3f0e in main (argc=26, argv=0xbffffba4, envp=0xbffffc10) at /home/ianc/devel/qemu.git/vl.c:3162
> > >
> > > I suspect this is because xen_init() (which sets xch) is never called,
> > > perhaps because it causes the accelerator to always be tcg? If I use
> > > "-machine xenfv,accel=xen" then it works as expected, -M continues to
> > > work too AFAICT.
> > >
> > > It would be nice to retain the behaviour of defaulting to accel=xen for
> > > machine=xenfv. (to be honest I can't quite see where this behaviour came
> > > from, nor what about this patch changes it...)
> >
> > Well, first of all I think this revealed a Xen bug because it crashes
> > when you try to run xenfv with an inappropriate accelerator, no? What is
> > the result of -machine xenfv,accel=tcg or, without my patch, -M xenfv
> > -machine accel=tcg?
>
> Unsurprisingly it crashed...
>
> I'm not sure if this is a bug in the xen side of simply a case of
> providing enough rope. I didn't really follow the threads which resulted
> in the accel stuff all that closely so I'm not sure what the intention
> was, it seems to me that the accel option is invalid with certain
> machine types though.

This options was here to initialise the xen stuff early in the QEMU
initilisation, and the purpose was to replace --enable-xen (and
--enable-kvm) at the same occasion.

But since most of the xen stuff have been put in xen_hvm_init, called
by machine{xenfv}->init(), it's could make sense to call xen_init from
xen_hvm_init, or just check if it have been called.

> I suppose in theory -machine xenpv,accel=kvm might result in xenner or
> something, accel=xenpv,tcg I'm less sure about (perhaps xenner too?).
> For -machine xenfv I don't expect anything other than accel=xen makes
> much sense.

But a -machine pc,accel=xen could result in a xenfv machine. We already
use the pc machine for xenfv with few difference. So a -machine
pc,accel=xen:kvm could use the hypervisor present on the machine :)
even if it's not realy useful in many case.


-- 
Anthony PERARD