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Re: [Xen-devel] [Patch V2] expand x86 arch_shared_info to support linear p2m list

On 12/01/2014 05:28 PM, Jan Beulich wrote:
On 01.12.14 at 15:33, <JGross@xxxxxxxx> wrote:
On 12/01/2014 02:37 PM, Jan Beulich wrote:
On 01.12.14 at 14:11, <JGross@xxxxxxxx> wrote:
On 12/01/2014 12:29 PM, Jan Beulich wrote:
On 01.12.14 at 12:19, <david.vrabel@xxxxxxxxxx> wrote:
On 01/12/14 10:15, Jan Beulich wrote:
On 01.12.14 at 10:29, <JGross@xxxxxxxx> wrote:
The x86 struct arch_shared_info field pfn_to_mfn_frame_list_list
currently contains the mfn of the top level page frame of the 3 level
p2m tree, which is used by the Xen tools during saving and restoring
(and live migration) of pv domains and for crash dump analysis. With
three levels of the p2m tree it is possible to support up to 512 GB of
RAM for a 64 bit pv domain.

A 32 bit pv domain can support more, as each memory page can hold 1024
instead of 512 entries, leading to a limit of 4 TB.

To be able to support more RAM on x86-64 switch to a virtual mapped
p2m list.

This patch expands struct arch_shared_info with a new p2m list virtual
address, the root of the page table root and a p2m generation count.
The new information is indicated by the domain to be valid by storing
~0UL into pfn_to_mfn_frame_list_list. The hypervisor indicates
usability of this feature by a new flag XENFEAT_virtual_p2m.

Right now XENFEAT_virtual_p2m will not be set. This will change when
the Xen tools support the virtual mapped p2m list.

This seems wrong: XENFEAT_* only reflect hypervisor capabilities.
I.e. the availability of the new functionality may need to be
advertised another way - xenstore perhaps?

Xenstore doesn't work for dom0.

Shouldn't this be something the guest kernel reports using a ELF note bit?

When building a domain (either in Xen for dom0 or in the tools), the
builder may provide a linear p2m iff supported by the guest kernel and
then (and only then) can it provide a guest with > 512 GiB.

Yes, surely this flag could act as a kernel capability indicator (via
XENFEAT_dom0 already does. JÃrgen's final statement, however,
suggested to me that this is meant to be only consumed by kernels.

Yes. The p2m list built by the domain builder is already linear. It may
just be to small to hold all entries required e.g. for Dom0.

It's Xen-tools and kdump which have to deal with the linear p2m list.
So the guest kernel has to be told if it is allowed to present the
linear list instead of the 3-level tree at pfn_to_mfn_frame_list_list.

As this is true for Dom0 as well, this information must be given by the

I'm aware that XENFEAT_* is only used for hypervisor capabilities up to
now. As the Xen tools are tightly coupled to the hypervisor I don't see
why the features can't express the capability of the complete Xen
installation instead. Would you prefer introducing another leaf for
that purpose (submap.idx == 1) ?

That wouldn't change the odd situation of reporting a capability of
another component. That's even more of a problem for the Dom0
case, where the affected tool (kdump) isn't even under our control
(and shouldn't be).

But in the end - what's wrong with always (or conditionally upon a
CONFIG_* option and/or command line parameter and/or memory
size) filling both the old and new shared info fields? A capable tool
can determine whether the new one is valid, and an incapable tool
won't work on huge memory configs anyway.

Okay, but this would require another way of reporting the validity of
the linear p2m list anchor, as setting pfn_to_mfn_frame_list_list to
an invalid value is no longer an option then.

As the shared info page is always zeroed when the domain is built we
could use a value different from 0 of e.g. the p2m_generation member
as an indicator for the validity.

Wouldn't both of the other new fields be guaranteed non-zero
when used?

p2m_cr3 yes, I suppose (mfn 0 is never part of a guest). I'm not sure
about p2m_vaddr. A paravirtualized OS could choose to put the p2m list
at virtual address 0.


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