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

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)
        if (req) {
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

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


     -Don Slutz

+        } else {
+            handle_ioreq(req);
+        }

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