Re: [Xen-devel] [PATCH] libxc: remove stale error check for domain size in xc_sr_save_x86_hvm.c

[Juergen Gross <jgross@xxxxxxxx>]

On 26/09/17 09:41, Andrew Cooper wrote:
> On 26/09/2017 08:33, Juergen Gross wrote:
>> Long ago domains to be saved were limited to 1TB size due to the
>> migration stream v1 limitations which used a 32 bit value for the
>> PFN and the frame type (4 bits) leaving only 28 bits for the PFN.
>>
>> Migration stream V2 uses a 64 bit value for this purpose, so there
>> is no need to refuse saving (or migrating) domains larger than 1 TB.
>>
>> Remove the related test in xc_sr_save_x86_hvm.c
>>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> 
> Sorry, but this still serves a very real purpose.  Without it, you will
> crash a 32bit toolstack trying to migrate a large domain, due to virtual
> address space exhaustion.
> 
> Things are hopefully better now that we don't unconditionally map the
> m2p, but iirc, 1.2T domains were the tipping point last time I tried.
> 
> I didn't come up with any more clever way of limiting the size, which is
> why this check persisted from the legacy code.

BTW: this test is really confusing!

You don't want to limit the pfns to 28 bits due to the type mask, but
you just want to limit the domain size due to a (potential) lack of
host resources.

So the least to do would be a test for the size without using
XEN_DOMCTL_PFINFO_LTAB_MASK together with a comment _why_ the test is
needed.


Juergen

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