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Re: [Xen-devel] [PATCH 1/2] xen, libxc: init msix addr/data with value from qemu via hypercall

>>> On 08.05.13 at 12:00, Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> wrote:

> On 2013-05-08 17:39, Jan Beulich wrote:
>>>>> On 08.05.13 at 10:17, Zhenzhong Duan<zhenzhong.duan@xxxxxxxxxx>  wrote:
>>> Accelerated msix entry is initialized to zero when msixtbl_pt_register is
>>> called. This doesn't match the value from qemu side, although pirq may
>>> already
>>> be mapped and binded in qemu side. Kernel will get wrong value when reading
>>> msix info.
>>> Signed-off-by: Zhenzhong Duan<zhenzhong.duan@xxxxxxxxxx>
>>> Tested-by: Yuval Shaia<yuval.shaia@xxxxxxxxxx>
>> I appreciate this needing to change, but it is a no-go to expose an
>> implementation detail of the hypervisor (number of accelerated
>> entries being 3) trough a hypercall interface (and even less so by
>> scattering around literal 3-s).
> I presume you mean msi_ad[3]. msi_ad[3] is addr_lo, addr_high and data.
> Not related to accelerated entries count.
> or others?

Oh, right you are. But then nevertheless give this meaningful
names in the hypercall interface (e.g. addr_lo, addr_hi, and data,
or just [64-bit] addr and [32-bit] data) rather than following the
bad practice in vmsi.c.

>> Please work towards a different solution, leaving the tool stack
>> agnostic to the number of accelerated entries. And if at all
>> possible, arrange for the patch to be split into tool stack and
>> hypervisor pieces, such that they can be applied independently
>> (and in either order).
> sure, will do it after above question is clear.

With the above it's going to be difficult to split the two pieces.

But of course I still don't really understand why all of the sudden
this needs to be passed in rather than being under the full control
of the hypervisor at all times. Perhaps this is related to me not
understanding why the kernel would read these values at all:
There's no other place in the kernel where the message would
be read before first getting written (in fact, apart from the
use of __read_msi_msg() by the Xen code, there's only one
other user under arch/powerpc/, and there - according to the
accompanying comment - this is just to save away the data for
later use during resume).


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