Re: [Xen-devel] [PATCH 1/1] xen-hvm.c: Add support for Xen access to vmport

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 1 Oct 2014, Slutz, Donald Christopher wrote:
> On 09/30/14 06:35, Stefano Stabellini wrote:
> > On Mon, 29 Sep 2014, Don Slutz wrote:
> >> On 09/29/14 06:25, Stefano Stabellini wrote:
> >>> On Mon, 29 Sep 2014, Stefano Stabellini wrote:
> >>>> On Fri, 26 Sep 2014, Don Slutz wrote:
> >>>>> This adds synchronisation of the vcpu registers
> >>>>> between Xen and QEMU.
> >>>>>
> >>>>> Signed-off-by: Don Slutz <dslutz@xxxxxxxxxxx>
> >>>> [...]
> >>>>
> >>>>> diff --git a/xen-hvm.c b/xen-hvm.c
> >>>>> index 05e522c..e1274bb 100644
> >>>>> --- a/xen-hvm.c
> >>>>> +++ b/xen-hvm.c
> >>>>> @@ -857,14 +857,48 @@ static void cpu_handle_ioreq(void *opaque)
> >>>>>          handle_buffered_iopage(state);
> >>>>>        if (req) {
> >>>>> +#ifdef IOREQ_TYPE_VMWARE_PORT
> >>>> Is there any reason to #ifdef this code?
> >>>> Couldn't we just always build it in QEMU?
> >> This allows QEMU 2.2 (or later) to build on a system that has Xen 4.5
> >> or earlier installed; and work.
> > I would rather remove the #ifdef from here and add any necessary
> > compatibility stuff to include/hw/xen/xen_common.h.
> >
> 
> Ok, will do.
> 
> >>>>> +        if (req->type == IOREQ_TYPE_VMWARE_PORT) {
> >>>> I think it would make more sense to check for IOREQ_TYPE_VMWARE_PORT
> >>>> from within handle_ioreq.
> >>>>
> >> Ok, I can move it.
> >>
> >>
> >>>>> +            CPUX86State *env;
> >>>>> +            ioreq_t fake_req = {
> >>>>> +                .type = IOREQ_TYPE_PIO,
> >>>>> +                .addr = (uint16_t)req->size,
> >>>>> +                .size = 4,
> >>>>> +                .dir = IOREQ_READ,
> >>>>> +                .df = 0,
> >>>>> +                .data_is_ptr = 0,
> >>>>> +            };
> >>> Why do you need a fake req?
> >> To transport the 6 VCPU registers (only 32bits of them) that vmport.c
> >> needs to do it's work.
> >>
> >>> Couldn't Xen send the real req instead?
> >> Yes, but then a 2nd exchange between QEMU and Xen would be needed
> >> to fetch the 6 VCPU registers.  The ways I know of to fetch the VCPU 
> >> registers
> >> from Xen, all need many cycles to do their work and return
> >> a lot of data that is not needed.
> >>
> >> The other option that I have considered was to extend the ioreq_t type
> >> to have room for these registers, but that reduces by almost half the
> >> maximum number of VCPUs that are supported (They all live on 1 page).
> > Urgh. Now that I understand the patch better is think it's horrible, no
> > offense :-)
> 
> None taken.
> 
> > Why don't you add another new ioreq type to send out the vcpu state?
> > Something like IOREQ_TYPE_VCPU_SYNC_REGS? You could send it to QEMU
> > before IOREQ_TYPE_VMWARE_PORT. Actually this solution looks very imilar
> > to Alex's suggestion.
> >
> 
> I can, it is just slower.  This would require 2 new types.  1 for regs to
> QEMU, 1 for regs from QEMU.  So instead of 1 round trip (Xen to QEMU
> to Xen), you now have 3 (Xen to QEMU (regs to QEMU) to Xen, Xen to
> QEMU (PIO) to Xen, Xen to QEMU (regs from QEMU) to Xen).

This is not an high performance device, is it?

In any case I agree it would be better to avoid it.
I wonder if we could send both ioreqs at once from Xen and back from
QEMU. Or maybe append the registers to IOREQ_TYPE_VMWARE_PORT, changing
the size of ioreq_t only for this ioreq type.
Another maybe simpler possibility would be introducing a union in
ioreq_t with the registers.
Anything would be OK for me but I would like to see the registers being
passes explicitely by Xen rather than "hiding" them into other ioreq
fields.


> >>>    If any case you should spend a
> >>> few more words on why you are doing this.
> >>>
> >> Sure.  Will add some form of the above as part of the commit message.
> >>
> >>>>> +            if (!xen_opaque_env) {
> >>>>> +                xen_opaque_env = g_malloc(sizeof(CPUX86State));
> >>>>> +            }
> >>>>> +            env = xen_opaque_env;
> >>>>> +            env->regs[R_EAX] = (uint32_t)(req->addr >> 32);
> >>>>> +            env->regs[R_EBX] = (uint32_t)(req->addr);
> >>>>> +            env->regs[R_ECX] = req->count;
> >>>>> +            env->regs[R_EDX] = req->size;
> >>>>> +            env->regs[R_ESI] = (uint32_t)(req->data >> 32);
> >>>>> +            env->regs[R_EDI] = (uint32_t)(req->data);
> >>>>> +            handle_ioreq(&fake_req);
> >>>>> +            req->addr = ((uint64_t)fake_req.data << 32) |
> >>>>> +                (uint32_t)env->regs[R_EBX];
> >>>>> +            req->count = env->regs[R_ECX];
> >>>>> +            req->size = env->regs[R_EDX];
> >>>>> +            req->data = ((uint64_t)env->regs[R_ESI] << 32) |
> >>>>> +                (uint32_t)env->regs[R_EDI];
> >>>> This code could be moved to a separate helper function called by
> >>>> handle_ioreq.
> >>>>
> >> Ok.
> >>
> >>     -Don Slutz
> >>
> >>>>> +        } else {
> >>>>> +            handle_ioreq(req);
> >>>>> +        }
> >>>>> +#else
> >>>>>            handle_ioreq(req);
> >>>>> +#endif
> 

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel