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

["Slutz, Donald Christopher" <dslutz@xxxxxxxxxxx>]

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).

     -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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