Mapping pages from dom0

Mapping pages from dom0

[Jacob Gorm Hansen]

hi,

I am mapping some pages in a domU from dom0 userspace. Doing that once I 
know the page numbers is fairly trivial, but what is the recommended way 
of communicating one or more page numbers from domU to dom0, hopefully 
without having to modify xend?

Right now I just copy&paste from the console, but that seems uncool 
somehow ;-)

thanks,
Jacob


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Re: Mapping pages from dom0

[Mark Williamson]

> I am mapping some pages in a domU from dom0 userspace. Doing that once I
> know the page numbers is fairly trivial, but what is the recommended way
> of communicating one or more page numbers from domU to dom0, hopefully
> without having to modify xend?

Hrrrm.  There was talk of a simple generic control message that could be used 
for this sort of thing but I'm not sure if it's been implemented.

> Right now I just copy&paste from the console, but that seems uncool
> somehow ;-)

There's always the network, although that's kinda gross ;-)

Cheers,
Mark


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Re: Mapping pages from dom0

[Jacob Gorm Hansen]

Mark Williamson wrote:
>>I am mapping some pages in a domU from dom0 userspace. Doing that once I
>>know the page numbers is fairly trivial, but what is the recommended way
>>of communicating one or more page numbers from domU to dom0, hopefully
>>without having to modify xend?
> 
> 
> Hrrrm.  There was talk of a simple generic control message that could be used 
> for this sort of thing but I'm not sure if it's been implemented.

Sad. I tried sticking this at the last four bytes of the shared_info 
page, but apparently that one gets cleared every now and then :-(

Jacob


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Re: Mapping pages from dom0

[Jacob Gorm Hansen]

Jacob Gorm Hansen wrote:
> Mark Williamson wrote:
> 
>>> I am mapping some pages in a domU from dom0 userspace. Doing that once I
>>> know the page numbers is fairly trivial, but what is the recommended way
>>> of communicating one or more page numbers from domU to dom0, hopefully
>>> without having to modify xend?
>>
>>
>>
>> Hrrrm.  There was talk of a simple generic control message that could 
>> be used for this sort of thing but I'm not sure if it's been implemented.
> 
> 
> Sad. I tried sticking this at the last four bytes of the shared_info 
> page, but apparently that one gets cleared every now and then :-(

Anyway, for now I guess I can just extend the shared_info_t struct and 
things should work, but in the longer run it might be nice to have some 
kind of simple mechanism for doing this stuff.

Jacob


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RE: Mapping pages from dom0

[Ian Pratt]

> > Hrrrm.  There was talk of a simple generic control message 
> that could be used 
> > for this sort of thing but I'm not sure if it's been implemented.
> 
> Sad. I tried sticking this at the last four bytes of the shared_info 
> page, but apparently that one gets cleared every now and then :-(

The last half of the shared_info page is the control message ring, so
this won't work.

Ian


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Re: Mapping pages from dom0

[John L Griffin]

> I am mapping some pages in a domU from dom0 userspace. Doing that once I 

> know the page numbers is fairly trivial, but what is the recommended way 

> of communicating one or more page numbers from domU to dom0, hopefully 
> without having to modify xend?

I am curious: I'm just starting to look into the mechanisms by which pages 
are shared between domains.  The above question surprised me, because I'd 
assumed there existed some sort of anonymous "I want to map a page that 
domain N has exported" method to bootstrap direct interdomain 
communication between unprivileged domains.

Looking through the code now, could someone please comment on the 
correctness (or, more importantly, uncorrectness) of the following 
statements?

* It is possible for two unprivileged domains to share pages, without 
needing any intervention from a privileged domain, as long as each side 
knows in advance the addresses of the pages to be shared?

* The grant_table_op hypervisor call is used by Domain-A to notify Xen 
that "yes, I want to allow Domain-B to map this page"?  Then, the 
update_va_mapping hypervisor call is used by Domain-B to map the page?

* grant_table_op cannot be used by Domain-A to revoke Domain-B's mapping 
of the page?  (I.e., all the "unmap grant ref" flag says is "cause all 
future Domain-B update_va_mapping calls, regarding mapping this page, to 
fail"?)

* Jacob's problem (dom0 needing to know the addresses of pages shared by a 
domU) is avoided during virtual network and virtual block device setup, 
because the pages to be shared are somehow either set up by Xen or shared 
through the "control interface" channel to Xend?

Thanks,
JLG



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Re: Mapping pages from dom0

[Mark A. Williamson]

> I am curious: I'm just starting to look into the mechanisms by which pages
> are shared between domains.  The above question surprised me, because I'd
> assumed there existed some sort of anonymous "I want to map a page that
> domain N has exported" method to bootstrap direct interdomain
> communication between unprivileged domains.

To map another domain's memory you have to know the machine address of the 
page frame in question.

> Looking through the code now, could someone please comment on the
> correctness (or, more importantly, uncorrectness) of the following
> statements?

Here goes...

> * It is possible for two unprivileged domains to share pages, without
> needing any intervention from a privileged domain, as long as each side
> knows in advance the addresses of the pages to be shared?

Not yet.  Grant tables should enable this but are not completed yet.  For now, 
you need to be a privileged domain to map another domain's memory.

> * The grant_table_op hypervisor call is used by Domain-A to notify Xen
> that "yes, I want to allow Domain-B to map this page"?  Then, the
> update_va_mapping hypervisor call is used by Domain-B to map the page?

It will be, when the grant tables stuff is finished.

> * grant_table_op cannot be used by Domain-A to revoke Domain-B's mapping
> of the page?  (I.e., all the "unmap grant ref" flag says is "cause all
> future Domain-B update_va_mapping calls, regarding mapping this page, to
> fail"?)

I think the plan is to enable domains to appeal to domain 0 if a mapping that 
should have been relinquished by another domain is not.  Some sort of 
algorithm is then to be used by management code in domain 0 to decide what to 
do about it.

> * Jacob's problem (dom0 needing to know the addresses of pages shared by a
> domU) is avoided during virtual network and virtual block device setup,
> because the pages to be shared are somehow either set up by Xen or shared
> through the "control interface" channel to Xend?

The control interface protocols for each of the device interfaces include a 
message from the frontend that tells Xend where the shared memory frame for 
the device interface's comms ring is placed (in machine addresses).  Xend 
then sends this address to the backend so it knows where the interface ring 
is.

Messages passed over a device interface ring contain machine addresses that 
the backend can use to map the actual IO destination

Cheers,
Mark


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