Re: [PATCH v4.2] libxc: Defer initialization of start_page for HVM guests

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 01/08/2016 09:30 AM, Juergen Gross wrote:
> On 08/01/16 15:19, Boris Ostrovsky wrote:
>> On 01/07/2016 11:57 PM, Juergen Gross wrote:
>>> On 07/01/16 23:19, Boris Ostrovsky wrote:
>>>> With commit 8c45adec18e0 ("libxc: create unmapped initrd in domain
>>>> builder if supported") location of ramdisk may not be available to
>>>> HVMlite guests by the time alloc_magic_pages_hvm() is invoked if the
>>>> guest supports unmapped initrd.
>>>>
>>>> So let's move ramdisk info initialization (along with a few other
>>>> operations that are not directly related to allocating magic/special
>>>> pages) from alloc_magic_pages_hvm() to bootlate_hvm().
>>>>
>>>> Since we now split allocation and mapping of the start_info segment
>>>> let's stash it, along with cmdline length, in xc_dom_image so that we
>>>> can check whether we are mapping correctly-sized range.
>>>>
>>>> We can also stop using xc_dom_image.start_info_pfn and leave it for
>>>> PV(H) guests only.
>>>>
>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>>> ---
>>>> v4:
>>>>    * See the last two paragraphs from commit message above
>>>>
>>>> v4.1:
>>>>    * Inverted testing of start_info_size in bootlate_hvm().
>>>>
>>>> v4.2
>>>>    * <facepalm> Actually do what I said I'd do in 4.1
>>>>
>>>>    tools/libxc/include/xc_dom.h |    2 +
>>>>    tools/libxc/xc_dom_x86.c     |  140
>>>> +++++++++++++++++++++++------------------
>>>>    2 files changed, 80 insertions(+), 62 deletions(-)
>>>>
>>>> diff --git a/tools/libxc/include/xc_dom.h b/tools/libxc/include/xc_dom.h
>>>> index 2460818..cac4698 100644
>>>> --- a/tools/libxc/include/xc_dom.h
>>>> +++ b/tools/libxc/include/xc_dom.h
>>>> @@ -71,6 +71,7 @@ struct xc_dom_image {
>>>>          /* arguments and parameters */
>>>>        char *cmdline;
>>>> +    size_t cmdline_size;
>>>>        uint32_t f_requested[XENFEAT_NR_SUBMAPS];
>>>>          /* info from (elf) kernel image */
>>>> @@ -91,6 +92,7 @@ struct xc_dom_image {
>>>>        struct xc_dom_seg p2m_seg;
>>>>        struct xc_dom_seg pgtables_seg;
>>>>        struct xc_dom_seg devicetree_seg;
>>>> +    struct xc_dom_seg start_info_seg; /* HVMlite only */
>>> Instead of adding HVM specific members here, you could make use of
>>> dom.arch_private and use just a local structure defined in xc_dom_x86.c.
>> I did consider this but since we already keep type-specific segments in
>> this structure (e.g. p2m_seg) decided to add an explicit segment for
>> HVMlite.
> But p2m_seg is accessed from multiple sources, while cmdline_size and
> start_info_seg would be local to xc_dom_x86.c
>
> BTW: thanks for the hint - I'll have a look whether p2m_seg can't be
> moved to arch_private...
>
>> Besides, I think to properly use it we'd need to add an arch hook and
>> IMHO it's not worth the trouble in this case.
> Why would you need another arch hook? Just add the arch_private_size
> member to struct xc_dom_arch and everything is set up for you. Look
> how it is handled for the pv case in xc_dom_x86.c

So it is already hooked up, I didn't notice that we do register 
xc_hvm_32, even though arch_private_size is 0.

This would be a type-specific area though, not arch-specific as the name 
implies. So perhaps xc_dom_image_x86 should be modified to include 
type-specific structures (via a union).

-boris