Re: Memory strategy in xen

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 03/12/13 13:48, Yunbin Wang wrote:
> Hi Andrew,
> Thanks for your reply.
>
> How does Xen map guest virtual address to machine physical address in
> PV mode?
>
> In PV mode, guests are aware of VMM, and know the actual physical
> memory address assigned to them, so do they map guest virtual address
> to guest physical address first, and then map physical address to
> machine physical, OR they map virtual address to machine physical
> directly?
>
> Thanks,
> Yunbin Wang

Please do not drop xen-devel from CC, and please do not top-post.

Xen does not keep track of guest virtual address - that is the guests
job to do.  Xen maintains the pagetables, but the PV guest has to ask
Xen to update a PTE, and what value to update it to.

PV guests are aware that their physical address space is not contiguous,
and have to take this into account for memory management purposes.

~Andrew

>
>
> 2013/11/18 Andrew Cooper <andrew.cooper3@citrix.com
> <mailto:andrew.cooper3@citrix.com>>
>
>     On 17/11/13 00:26, Yunbin Wang wrote:
>>     Hi all,
>>     I'm a newer to xen. But now I need to do research about live
>>     migration in xen. And now I'm confused with some questions:
>>     1. Is shadowing page used in both paravirtualization and hvm
>>     mode? Since from source code xc_domain_save.c, I see the
>>     following code to get dirty memory bitmap,
>>
>>     frc = xc_shadow_control(
>>                         xch, dom, XEN_DOMCTL_SHADOW_OP_PEEK,
>>     HYPERCALL_BUFFER(to_skip),
>>                         dinfo->p2m_size, NULL, 0, NULL);
>
>     From domctl.h
>
>     /* Log-dirty bitmap operations. */
>      /* Return the bitmap and clean internal copy for next round. */
>     #define XEN_DOMCTL_SHADOW_OP_CLEAN       11
>      /* Return the bitmap but do not modify internal copy. */
>     #define XEN_DOMCTL_SHADOW_OP_PEEK        12
>
>     Shadow and logdirty are related, but not the same.  Shadow is
>     certainly not expected to be used with PV guests (although a PV
>     guest specifying autotranslate will be run with shadowing enabled).
>
>     ~Andrew
>
>>
>>
>>     2. The strategy about memory allocation in xen, I know that dom0
>>     will get all available memory at first, and xen will allocate
>>     memory via balloon strategy to domU.
>>
>>     And now I need to know how balloon works?
>>
>>     Can anyone give me some ideas about questions above.
>>
>>     Thanks,
>>     Yunbin Wang
>>
>>
>>     _______________________________________________
>>     Xen-devel mailing list
>>     Xen-devel@lists.xen.org <mailto:Xen-devel@lists.xen.org>
>>     http://lists.xen.org/xen-devel
>
>


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