Re: [Xen-devel] [RFC PATCH v2 3/3] tools, libxl: handle the iomem parameter with the memory_mapping hcall

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Ian,

On 03/13/2014 03:27 PM, Ian Campbell wrote:
> On Mon, 2014-03-10 at 09:25 +0100, Arianna Avanzini wrote:
>> Currently, the configuration-parsing code concerning the handling of the
>> iomem parameter only invokes the XEN_DOMCTL_iomem_permission hypercall.
>> This commit lets the XEN_DOMCTL_memory_mapping hypercall be invoked
>> after XEN_DOMCTL_iomem_permission when the iomem parameter is parsed
>> from a domU configuration file, so that the address range can be mapped
>> to the address space of the domU.
>> NOTE: the added code is still common to both x86 and ARM; it also
>>       implements a simple 1:1 mapping that could clash with the domU's
>>       existing memory layout if the range is already in use in the
>>       guest's address space.
> 
> In that case you need to CC the x86 maintainers (Jan, Keir, Tim) here.
> It doesn't seem to me that this is going to be the correct thing to do
> for either x86 PV or x86 HVM guests.

From my reply on V1, the XEN_DOMCTL_memory_mapping hypercall is called
by QEMU for HVM. I was unable to define where this call is made by PV.

>> Signed-off-by: Arianna Avanzini <avanzini.arianna@xxxxxxxxx>
>> Cc: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
>> Cc: Paolo Valente <paolo.valente@xxxxxxxxxx>
>> Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>> Cc: Julien Grall <julien.grall@xxxxxxxxxx>
>> Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
>> Cc: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
>> Cc: Eric Trudeau <etrudeau@xxxxxxxxxxxx>
>> Cc: Viktor Kleinik <viktor.kleinik@xxxxxxxxxxxxxxx>
>> ---
>>  tools/libxl/libxl_create.c | 17 +++++++++++++++++
>>  1 file changed, 17 insertions(+)
>>
>> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
>> index a604cd8..6c206c3 100644
>> --- a/tools/libxl/libxl_create.c
>> +++ b/tools/libxl/libxl_create.c
>> @@ -1099,6 +1099,23 @@ static void domcreate_launch_dm(libxl__egc *egc, 
>> libxl__multidev *multidev,
>>                   "failed give dom%d access to iomem range 
>> %"PRIx64"-%"PRIx64,
>>                   domid, io->start, io->start + io->number - 1);
>>              ret = ERROR_FAIL;
> 
> Please add a continue here and drop the else brining the remainder of
> the added code out an indentation level.
> 
> The existing error handling in this function seems very sketchy to me,
> there's a bunch of places where we set ret but then carry on regardless,
> more than likely overwriting ret again. It's possible that a bunch of
> goto error_out's should be added.
> 
>> +        } else {
>> +            /*
>> +             * NOTE: the following code is still common to both x86
>> +             *       and ARM; it also implements a simple 1:1 mapping
>> +             *       that could clash with the domU's existing memory
>> +             *       layout if the range is already in use in the
>> +             *       guest's address space.
> 
> The right thing to do here is to add a guest address field to
> libxl_iomem_range and to extend the xl parse to accept
>       iomem = [ "MFN,NR@PFN" ] syntax
> where @PFN is optional and the default is 1:1.

I disagree with this solution, the user doesn't know the guest layout.
How is it possible to let him choose the pfn?

I think, for now the best solution is to expose the same memory layout
as the host when iomem is set.

Regards,

-- 
Julien Grall

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