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

[Don Slutz <dslutz@xxxxxxxxxxx>]

On 10/01/14 05:20, Stefano Stabellini wrote:
> 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?
Depends on how you view mouse device speed.

> 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.
No idea how do do this since this is an array in a shared page.

> 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.
I considered a union, but this can be a issue with older QEMU
and newer Xen.  Since it appears that you care, I will add a new
struct for this.  The problem is that it is a "union" and so must
match on the common fields.

   -Don Slutz



> > > > >     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

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